What kind of arthropod/animal is this?

What kind of arthropod/animal is this?

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Can anyone identify this strange creature? I live in a very urban part of Montreal, Canada. It wasn't moving (seemed dead) and although the image is huge, it wasn't as big as it seems (a couple of centimeters).

It's a type of centipede.

Based on the long legs, I would bet on something like that:

Or at least a Scutigeromorpha

I live in Japan and here they are called "gejigeji". As mentioned, they are a member of the centipede family and are as creepy as hell. I live in a newly-built house, so thankfully there are no cockroaches, but every now and again one of these little blighters will scurry across my floor in my peripheral vision and scare the bejesus out of me.

They are not dangerous (at least not to humans). I have been told they eat the larvae of other insects, like cockroaches so they are not so bad really. Also, and I assume this is just a Japanese myth, but it's said they hunt in pairs.

It is a common house centipede. Apparently they're everywhere but I've never seen one before.

Arthropod Eye & Vision

Arthropods possess simple as well as compound eyes the latter evolved in Arthropods and are found in no other group of animals. Insects that possess both types of eyes are considered to be the most successful animals on earth.

VISION IN CRUSTACEA (Prawn) – The Compound Eye

Crustacea includes prawns, crabs, lobsters, shrimps, barnacles, water fleas etc., which possess a pair of compound eyes for vision.

Prawn possesses a pair of large stalked hemispherical eyes on the anterior side of cephalothorax below the rostrum. Each eye is composed of a large number of independent visual units called ommatidia which are connected to the optic nerve. An ommatidium is divisible into the outer dioptrical region for receiving and focusing light rays and the inner sensory region for perceiving light and sending the nerve impulse to the brain, which analyses the impulses as image of the object.

The cuticle on the surface is modified as cornea over the ommatidia and gives the eye necessary protection and also allows the light rays to enter the eye. Below the cornea, a pair of corneagen cells secretes fresh cornea in case of wear and tear. A lens-like crystalline cone is located beneath the corneagen cells and serves to focus light rays inwards. The crystalline cone is surrounded by four cone cells or Vitrellae that serve to provide nourishment to the cone.

Next layer is of sensory cells called Rhabdomes which are elongated and transversely striated and are sensory in function. Seven retinalcells that surround the rhabdome and encircle it provide it nutrition and protection. Chromatophores are pigment cells which are responsible for separating one ommatidium from the other so that they remain as independent units. They are located around the cone cells and retinal cells and can shrink or expand to increase or decrease the intensity of light entering the eye.


The compound eye is incapable of giving distant vision and sharp vision but is efficient in picking up motion and in providing 360° view, as it is large globular and mounted on a movable stalk. Each ommatidium is capable of producing an independent image of a small part of the object seen and not the entire object. All these small images are combined in the brain to form a complete image of the object that is made of small dots or mosaic of dots and hence it is called mosaic vision. The range of the compound eye is not more than a foot and hence no single ommatidium can perceive the entire object. Movement of the objects can be detected much more efficiently by the compound eye because as the object passes in front of the eye, the ommatidia switch on and off according to their location in relation to the object . This characteristic of the compound eye helps the animal in detecting the movement of the predators and escape before the latter can strike.

Another characteristic feature of the compound eye is its high flicker fusion rate, which means it can perceive action as successive independent frames of images and not as a continuous motion. The flicker fusion rate of the compound eye is about 50 frames per second as compared to 12-15 frames per second of human eye. By perceiving motion the compound eye helps arthropods to escape from predators.

The Apposition Image

This is perceived in bright light, when pigment cells in the dioptrical and sensory regions spread and completely separate the ommatidia from each other, so that the angle of vision of an ommatidium is only 1 degree and light rays coming directly from the front can only enter the ommatidium, whereas the light rays coming at an angle are absorbed by the pigment before they can reach rhabdomes. The image formed in brain is a mosaic of several dots, each one of which is formed by an ommatidium. Each ommatidium uses only a tiny portion of the total field of vision and then in brain these tiny images are grouped together to form a single image of the object. Since each dot is clearly separated from the other, it is called mosaic or apposition image. The sharpness of the image depends on the number of ommatidia and their isolation from one another.

The Superposition Image

This type of vision occurs in dim light in nocturnal arthropods. The pigment cells shrink to allow more light into the eye, so that the ommatidia no longer remain optically isolated from one another, enabling even oblique light rays to strike one or more ommatidia. This results in overlapping of the adjacent blotches of images formed by different ommatidia. This is called superposition image because overlapping images are formed in the brain. This image is not sharp but hazy because of overlapping images.

VISION IN ARACHNIDA (Scorpion) – The simple eye

Scorpion belongs to the class Arachnida and possesses only simple eyes. It has a pair of large median indirect eyes and three pairs of lateral direct eyes which function in different ways in different situations.

The Median Indirect eyes: The median eyes are large convex and covered with the thick cuticle that forms cornea or lens. The hypodermis forms a thick vitreous body that nourishes the lens. The sensory rhabdomes point backwards towards the reflecting layer called tapetum. The rhabdomes are surrounded by many sensory retinal cells which transmit nerve impulses to the optic nerve and then to the brain. Median eyes of scorpion are used for vision in the night or in dark places because the dim light entering the eye is reflected by the tapetum to strike the rhabdomes again to form vision.

The Lateral direct eyes: Lateral eyes are small in size, 3 pairs and located on the lateral sides of prosoma. This eye is covered externally by a biconvex lens formed from the transparent cuticle. The epidermis forms a thinner vitreous body under the lens. Inside the eye cup are several rhabdomes which point directly towards the source of light as the tapetum is absent in these eyes. Each rhabdome is connected on the posterior end to a sensory retinal cell that is connected to the nerve. The lateral eyes are used to provide vision in day time or in bright light.

VISION IN INSECTA (Cockroach) – Simple as well as Compound Eyes

Insects possess one pair of compound eyes and 1-3 simple eyes or ocelli on top of the head. In cockroach the ocelli are rudimentary.

The Insect Compound Eyes: The compound eyes are sessile in the form of convex brownish-black, kidney-shaped structures on the lateral sides of head. Each eye contains about 2,000 ommatidia, similar in structure to those already described earlier.

The pigments separating ommatidia are not retractable in the eyes of cockroach since the animal is nocturnal and spends daytime in dark places. But the eye produces mosaic vision similar to the crustaceans. Compound eyes are specially adapted to perceive movements of objects. The insect compound eye is advanced structure because the number of ommatidia in insect eyes increases giving the eye sharpness of vision. Also the distance of vision increases in predatory insects and fast flying insects.

The Insect Ocelli. Ocelli are simple eyes, more or less similar to the simple eyes of arachnids and provide the eye with distant vision. Ocelli also give nocturnal vision to night flying insects, which find their way by aligning them at an angle with the moon or stars. By possessing both types of eyes, insects enjoy both types of visions, namely detection of movement with compound eyes and distant vision with simple eyes or ocelli.

You know the first one. Arthropods all have exoskeletons. Exoskeletons are hard outer shells made of chitin. While you have an endoskeleton, a crab has a tough shell that protects it from the outside world. Next on the list are the arms and legs. They have jointed appendages. That's what the name arthropod means. jointed leg. Inside those joints and exoskeletons are muscles that help the organisms move.

Not all exoskeletons are the same. While they may all have chitin, a shell created by the epidermis, crustaceans have an extra layer that is calcified. That calcification makes it much sturdier and much heavier. Arthropods also have very advanced sense organs. You are probably familiar with the faceted eyes of flies and antennae on insects. Those are great examples of how arthropods are prepared to interact with the world. They also have open circulatory systems. These systems circulate nutrients throughout the inside of that exoskeleton so the muscles receive all the energy needed to move quickly.

Physiological Processes of Flatworms

Free-living species of flatworms are predators or scavengers, whereas parasitic forms feed from the tissues of their hosts. Most flatworms have an incomplete digestive system with an opening, the “mouth,” that is also used to expel digestive system wastes. Some species also have an anal opening. The gut may be a simple sac or highly branched. Digestion is extracellular, with enzymes secreted into the space by cells lining the tract, and digested materials taken into the same cells by phagocytosis. One group, the cestodes, does not have a digestive system, because their parasitic lifestyle and the environment in which they live (suspended within the digestive cavity of their host) allows them to absorb nutrients directly across their body wall. Flatworms have an excretory system with a network of tubules throughout the body that open to the environment and nearby flame cells, whose cilia beat to direct waste fluids concentrated in the tubules out of the body. The system is responsible for regulation of dissolved salts and excretion of nitrogenous wastes. The nervous system consists of a pair of nerve cords running the length of the body with connections between them and a large ganglion or concentration of nerve cells at the anterior end of the worm here, there may also be a concentration of photosensory and chemosensory cells ([Figure 1]).

Figure 1: This planarian is a free-living flatworm that has an incomplete digestive system, an excretory system with a network of tubules throughout the body, and a nervous system made up of nerve cords running the length of the body with a concentration of nerves and photosensory and chemosensory cells at the anterior end.

Since there is no circulatory or respiratory system, gas and nutrient exchange is dependent on diffusion and intercellular junctions. This necessarily limits the thickness of the body in these organisms, constraining them to be “flat” worms. Most flatworm species are monoecious (hermaphroditic, possessing both sets of sex organs), and fertilization is typically internal. Asexual reproduction is common in some groups in which an entire organism can be regenerated from just a part of itself.

What are Some Bioluminescent Animals? (with pictures)

Bioluminescent animals can be found in at least half a dozen animal phyla. This includes bioluminescent cnidarians (jellyfish, coral, and sea-pens), ctenophores ("comb jellies"), arthropods (fireflies, glow worms, certain fungus gnats, millipedes, and centipedes), certain annelids, one species of snail, marine molluscs including certain clams, nudibranchs, octopuses, and squids, various fish, some brittle stars, a group of small crustaceans, all krill, 65 species of mushrooms, protists called dinoflagellates, and a large family of bioluminescent bacteria. The last three aren't actually bioluminescent animals, but they are bioluminescent organisms.

Bioluminescence occurs in certain animals where chemical energy (in the form of ATP) is converted into light energy, usually peaking around one portion of the spectrum, making it one color. Green is by far the most common color used by terrestrial bioluminescent animals, while blue is the favored color among bioluminescent animals in the sea. Every color on the spectrum has a bioluminescent animal or protein associated with it, but most colors are quite rare. The difference in favored colors on the land and sea exists because different colors stand out in each environment, and the visual systems of animals in each environment are tuned to the local colors.

There are five accepted theories on why bioluminescent animals exist. These are that bioluminescence can perform the functions of camouflage, attraction (of prey, predators of would-be predators, and mates), repulsion by way of confusion, communication between bioluminescent bacteria (quorum sensing), and rarely, the illumination of prey (used by the Black Dragonfish). It can be hard to explain why certain organisms are bioluminescent, while with others, the reasons may be obvious.

For instance, in some species, like fireflies, bioluminescence is so integrated with the organism that it is an integral part of its lifestyle -- firefly larvae use it to repel predators, while adults use it to attract prey and signal to mates. Turn on a light bulb in an insect-infested area and you'll see the benefit of luminescence to attracting prey. Fireflies are extremely efficient at converting chemical energy into light -- they do it with an efficiency of 90%. In contrast, a typical incandescent light bulb is only 10% efficient.

Another common group of bioluminescent organisms are bioluminescent fungi. These glow green to attract nocturnal animals to aid in spore dispersal.

Michael is a longtime InfoBloom contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

Michael is a longtime InfoBloom contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

Six and Eight Legs

An insect’s body is divided into three main parts: the head the middle section, called the thorax and the end section, called the abdomen. A spider's body has two segments: a cephalothorax and an abdomen. Insects have a brain, a nervous system, a heart, a gut for digestion, and tubes called tracheae to breathe oxygen. They have two antennae and six legs, both of which have special organs on them to sense sound vibrations and movement, and to "taste" and "smell" food (although they don’t have taste buds and noses like we do). Spiders have eight legs and, in general, have "simple" eyes instead of the "compound" eyes that give many insects much better vision.

Phylum Arthropoda: Features and Classification (With Diagram)

Arthropoda represents a vast assemblage of animals. At least three quarters of a million species have been described. The phylum includes several large classes, but its impor­tance lies mainly in the fact that it contains the class insecta which by itself represents almost three quarters of all described animal species (Fig. 1.100).

No other phylum of animals can rival the arthropods in success, which is due to the tremendous adaptive diversity that has enabled them to survive virtually in every habitat. Their success as terrestrial animal is probably due to the evolution of water- conserving excretory systems and gaseous- exchange organs and the development of a desiccation-resistant impermeable epicuticle.

Greek: arthron, joint podos, feet.

Diagnostic Features of Phylum Arthropoda:

1. Segmented body with bilateral symme­try.

2. Anterior segments specialized to form a distinct head.

3. Body is externally covered with a thick, tough and non-living, chitinous cuticle, forming the exoskeleton.

4. A pair of externally jointed appendages is usually present in each segment.

5. Arthropods exhibit ecdysis or moulting. They shed off the old exoskeleton and a new one develops from the underlying epidermis.

6. Presence of musculature with distinct striped muscles. Muscles are attached to the inner surface of the skeletal system.

7. Body cavity is haemocoel. True coelom is vestigeal in adults.

8. Mouth and anus are present at the two terminal ends of the body.

9. Circulation is of the open-type (i.e. blood vessels open within haemocoel). Paired lateral ostia permit the passage of blood into the dorsal pulsating tube – the heart, from the surrounding pericardial sinus.

10. Haemocyanin is the respiratory pigment.

11. Arthropods possess two types of excre­tory organs—malpighian tubules and saccules (end sacs). Saccules take the name of the appendage with which it is associated, like coxal glands, green glands, maxillary glands and so forth.

12. Nervous system consists of a cerebral ganglionic mass connected to two ven­tral nerve cord with segmental ganglia.

13. Sense organs are sensillum (hair, bristle, seta, pit, peg etc.) and compound eyes. The compound eyes are composed of many long, cylindrical units (ommatidium), each of which contains all the visual elements.

14. Most arthropods are dioecious with sex­ual dimorphism. Eggs are centrolecithal and cleavage commonly superficial.

15. Development may be direct or indirect.

Scheme of Classification of Phylum Arthropoda:

Modern zoologists believe that there are probably four main lines of arthropod evolu­tion. These lines are represented by the extinct Trilobita and the three living- Chelicerata, Crustacea and Uniramia. The uniramia contains the flourishing insecta. The first three groups have marine origin, while uniramia appears to have evolved on land.

Evidences from comparative morpho­logy and embryology seems, that at least the uniramians and probably even all four of the above arthropod groups had a separate origin from different annelidan or near annelidan ancestors.

In view of this, Arthropoda should be considered as a super-phylum and Trilobita, Chelicerata, Crustacea and Uniramia should each be raised to phy­lum rank (as has been done by Barnes et al, 1993 Barnes, 1998).

However, in this text, the traditional rank of phylum has been retained for Arthropoda and the four lines of its evolution have been recognized as subphyla. The classification retained here is after the classificatory plan as outlined by Ruppert and Barnes, 1994.

Systematic Resume of Phylum Arthropoda:

Subphylum Trilobita (extinct):

Trilobites were abundant and widely distributed in Paleozoic seas. They reached their height during Cambrian and Ordovician period and disappeared at the end of Palaeozoic era. From fossil specimens about 3900 species have been described.

Subphylum Chelicerata (Greek: chele, talon cerata, horns)

1. Bilaterally symmetrical. Body shape varying from elongated to almost spheri­cal.

2. Body divided into an anterior cephalothorax or prosoma, which is wholly or partly covered by a dorsal carapace, and a posterior abdomen or opisthosoma without legs.

3. Appendages uniramous. Pro-somal appendages present, comprising of a pair of chelate ‘chelicerae’ (helps in feeding), one pair of chelate leg-like or feeler like ‘pedipalps’ (helps in various functions) and four pairs of walking legs.

4. Chelicerates are the only arthropods which lack antennae.

5. Mouth anteroventral. Gut straight. From the mid gut region arise two to many pairs of digestive diverticula which secrete enzymes that intracellularly digest and absorb food.

6. Median ocelli are present.

7. Development generally direct, juvenile with the full complement of limbs.

Chelicerata contains about 63,000 described species placed in three classes.

1. Aquatic chelicerates with five or six pairs of abdominal appendages modified as gills.

2. Twelve segmented abdomen is sub­divided into a seven segmented meso- soma and a five segmented metasoma.

3. A prominent spike like caudal spine or telson is present at the end of the body.

4. Compound eyes fairly developed.

1. Bottom dwellers, nocturnal, found in shallow coastal water and are commonly known as horse-shoe crabs.

2. Prosoma is covered by a large, horse­shoe shaped carapace.

3. Caudal spine is elongated, slender and pointed.

4. Abdominal segments fused and bear six pairs of appendages.

5. Lamellate gills or book gills are present in five pairs (on the appendages of ninth to thirteenth segments).

6. Excretion takes place by four pairs of coxal glands.

7. Development through trilobite larval stage.

This subclass contains 3 genera and four species.

Limulus (Fig. 1.101), Tachypleus, Carcinoscorpius

Subclass Eurypterida (Extinct):

This subclass is also known as Gigantostraca and comprises of the extinct giant arthropods. They were aquatic and existed from the Ordovician to the Permian period.

1. Except a few (secondarily aquatic) forms, the arachnida includes all living terres­trial chelicerates.

2. Predator arachnids use poison or silk in prey capture.

3. Prosoma un-segmented, usually covered dorsally by a solid carapace. The primi­tive abdomen is divided into a pre- abdomen and a post-abdomen.

4. Prosoma consists of a pair of chelicerae, a pair of pedipalps and four pairs of legs.

5. Respiratory organs are either book lungs or trachea.

6. The epicuticle is waterproof due to an external wax layer.

7. Excretion by malpighian tubules, coxal glands and nephrocytes.

8. Eyes usually simple. Compound eyes when present are degenerated.

9. Sexes separate, single or paired gonads that lie in the abdomen. Fertilization is internal.

The arachnids comprise of over 98% of living chelicerates and include over 62,000 species.

True scorpion (Buthus, Palamnaeus), Micro-whip scorpion (Koenenia), Pseudo-scorpion (Chelifer), True spi­ders (Aranea) (Fig. 1.102A), Mites and ticks (Ixodes (Fig. 1.102B), Sarcoptes (Fig. 1.102C), Argas).

1. Small, benthic marine animals common­ly known as sea spiders.

2. Opisthosoma much reduced.

3. The head or cephalon bears four eyes and at its anterior end a cylindrical pro­boscis.

4. A pair of palps, a pair of ovigerous legs and usually four pairs of walking legs are present.

5. No special organs for gas exchange and excretion are present.

6. Reproductive openings are multiple and are present on the ventral side of coxae (all legs in females, second and fourth pair in males).

7. Dioecious. Development usually through a larva called protonymphon.

More than 1,000 species have been described and placed in a single order.

Nymphon, Pallene, Decolopoda, Pycnogonum

Subphylum Crustacea (Latin: crusta, a rind or crust)

1. Primarily marine although several are freshwater (13%) and a few are terres­trial (3%).

2. Head bears five pairs of appendages which comprise of two pairs of antennae (first pair being the antennules), one pair of mandibles and two pairs of maxillae.

3. The cylindrical or leaf-shaped appen­dages are all typically biramous, the two branches are of different size and shape.

4. Exoskeleton often calcareous.

5. Respiration usually through gills, which are typically associated with the appendages.

6. Excretory organs are paired and com­posed of an end sac, an excretory canal and a short exit duct, all located in the head.

7. Head bears a pair of compound eyes, sometimes located on movable stalks and a small median dorsal naupliar eye.

8. Dioecious. Copulation and egg brooding are very common. Development through different larval stages like nauplius, zoea etc.

About 40,000 species of this subphylum are divided into eleven classes.

1. Marine animals with small, elongated, worm like and translucent body.

2. Body comprises of a short, carapace-less cephalothorax (head and first trunk seg­ment) and a long trunk of over 30 similar segments, each with a pair of leaf-like, lateral limbs.

3. They are carnivorous and the first pair of trunk appendages are modified as pre­hensile maxillipeds for feeding. Other trunk appendages help in swimming.

5. Hermaphrodite. Development still unknown.

They were first discovered in 1981 and is represented by nine species.

Examples: Lasionectes, Speleonectes (Fig. 1.104).

1. Bottom dwelling, marine animals and are detritus feeder.

2. Body small, elongated and cylindrical, terminating in a telson with a long furca.

3. The body is divided into a horseshoe- shaped head, thorax and abdomen, with­out any development of cephalothorax or carapace.

4. All eight pairs of thoracic limbs are iden­tical and similar to the second maxillae.

5. The eleven segmented abdomen lacks appendages, except the first which retains reduced limbs.

6. Although compound eyes are present they are blind as these eyes are buried in the head.

7. Hermaphrodite and development inclu­des metanauplius stage.

Discovered in 1955 and is represented by ten species.

Example: Hutchinsoniella (Fig. 1.105).

Class Branchiopoda (gill feet):

1. Small crustaceans mainly restricted to fresh water.

2. Trunk appendages are flattened leaf like structures

3. Coxa is provided with a flattened epipod that serves as a gill and hence the name “gill feet”.

4. First antenna and second maxilla are vestigeal.

5. The last abdominal segment bears a two terminal process called cercopods.

6. Excretion by maxillary glands or shell glands.

7. Branchiopods brood their eggs.

This class comprises of about 850 living species.

Subclass Calamanostraca:

1. Body composed of thorax with appenda­ges and abdomen without appendages.

2. Thorax covered by a carapace.

Triops (tadpole shrimp) (Fig. 1.106E).

1. Body enclosed within a laterally com­pressed carapace.

Lynceus (clam shrimp) (Fig. 1.106B), Daphnia

1. Trunk composed of 11 to 18 segments with appendages.

3. Eyes compound and stalked.

Artemia (brine shrimp) (Fig. 1.106D), Branchinecta (fairy shrimp) (Fig. 1.106C).

1. Ostracods are small crustaceans some­times referred to as mussel or seed shrimp. They are widely distributed in the sea and in all types of freshwater habitats.

2. Body enclosed within a hinged bivalve and often calcareous shell formed by the carapace.

3. Head large, forms half of the body volume and contains four appendages- antennules, antennae, mandibles and first maxillae.

4. Trunk reduced having no more than two pairs of appendages.

5. Gills absent. Gas exchange is integumen­tary.

Ostracods contain a total of 5,700 species divided in two subclasses.

1. Shell valves with an antennal notch.

2. Second antennae usually adapted for swimming.

3. Two pairs of trunk appendages.

1. Valves of the shell without an antennal notch.

2. One or two pairs of trunk appendages.

Cypris (Fig. 1.107B), Pontocypris, Candona, Cypridopsis.

1. Most copepods are aquatic and free living, and there are many parasitic species also.

2. Mostly small with cylindrical bodies.

3. Trunk composed of a thorax bearing five pairs of biramous appendages and a five segmented appendage-less abdomen.

4. The anterior end of the body is the head which is either rounded or pointed and with well- developed mouth parts and antennae.

5. First pair of antennae longer than second pair and held outstretched.

6. They lack a carapace and compound eyes, but the median naupliar eye is typical.

7. Absence of gills in free living copepods.

8. Excretion by maxillary glands.

About 8,400 species have been identified of which over 1,000 species are parasitic.

Cyclops (Fig. 1.107C), Ergasilus (parasite) (Fig. 1.107A), Diaptomus, Misophria, Harpacticus, Penella (parasite on flying fish).

1. Marine interstitial crustaceans with elon­gated, pigment less body.

2. Head is divided into a small anterior and a large posterior portion.

3. Trunk is made of ten segments of which the first five bear appendages, the first one being the maxilliped.

4. Only naupliar eye is present.

5. Sexes separate, development through nauplius stage.

Mystacocarida was first described in 1943 and twelve species have been identified.

1. Branchiurans are small, ectoparasites of marine and fresh water fishes.

2. Body dorsoventrally flattened.

3. A shield-like carapace covers the head and thorax.

4. Abdomen small, bilobed and un-segmented.

5. Both pairs of antennae reduced and modified for attachment.

6. The bases of the first pair of maxillae is modified into two large suckers (for attachment), the rest of the appendages being vestigial.

7. The four thoracic appendages well developed and used for swimming.

8. Presence of a pair of sessile compound eyes.

There are about 150 species of Branchiurans.

Examples: Argulus (Fig. 1.109), Dolops.

Class Pentastomida (five mouths):

1. Pentastomids are parasites that live within the lungs or nasal passage ways of vertebrates which include about 90% reptiles.

2. Body worm like and bears five short, anterior protuberances.

3. Four of these projections are leg-like bearing claws, while the central fifth pro­jection is a snout-like process bearing the mouth.

4. Body covered by a non-chitinous cuticle.

5. No circulatory, excretory and respiratory organs.

6. Sexes separate, fertilization internal, larval development requires an inter­mediate host.

There are about 90 parasitic species of pentastomids.

Cephalobaena (parasite on lung of a snake) (Fig. 1.110), Linguatula.

Class Tantulocarida:

1. Tantulocarids are minute ectoparasites of marine crustaceans.

2. The adult male remains permanently attached to the host by an oral disc.

3. Head without appendage and eye.

4. Thorax six segmented bearing five pairs of biramous limbs and a posterior uniramous one.

5. Abdomen two to six segmented and limbless. About twelve species have been identified under this class.

1. Cirripedes are either sessile or parasitic marine animals familiarly known as bar­nacles.

2. Body is poorly segmented and most lack an abdomen.

3. Body enclosed within a bivalved cara­pace.

4. Six pairs of biramous filamentous appen­dages are present.

5. Both pairs of antennae reduced or absent.

6. Gills are lacking and the excretory organs are maxillary glands.

7. Cirripedes development comprises of the nauplius larva that passes through a second larva, the cypris.

About 1,000 species have been identified.

Lepas (Goose barnacles) (Fig. 1.112B), Balanus (Acorn barnacles) (Fig. 1.112A), Dendrogaster, Sacculina (parasite), Verruca, Trypetesa

1. Malacostraca (the largest class of crustacea) body comprises of a head, an eight segmented thorax and a six segmented abdomen.

2. All the fourteen segments bear appen­dages.

3. Thorax may or may not be covered by a carapace.

4. The posterior thoracic limbs being walk­ing legs (pereiopods), the first five pairs of abdominal ones forming swimming organs (pleopods).

The foregut in most malacostracans is modified as a two-chambered stomach bearing triturating teeth and comb-like filtering setae.

6. Compound eyes present in most species.

7. The female and male gonopores open onto the fifth and eighth thoracic seg­ments, respectively.

This class comprises of about 23,000 species divided into three subclasses.

Subclass Phyllocarida:

1. Presence of seventh abdominal segment lacking appendages.

2. Foliaceous appendages present in thorax.

3. Thorax enclosed within a bivalve carapace

Subclass Hoplocarida (armed shrimp):

1. Marine crustaceans (about 300 species), called mantis shrimp that are highly specialized predators of fishes, crabs, shrimps and molluscs.

2. Body dorsoventrally flattened with a small shield like carapace that does not cover the last two thoracic segments.

3. The second pair of thoracic appendages large and sub-chelate which is adapted for capturing prey.

4. Pleopods well developed and bear fila­mentous gills.

5. First antennae with three flagella.

6. Compound eyes large, well developed and stalked.

Squilla (Fig. 1.113C), Gonodactylus, Lysiosquilla

Subclass Eumalacostraca:

1. Antennae without three flagella.

2. Seventh abdominal segment lacking.

Euphausia (Antarctic krill), Penaeus, Lucifer, Macro-brachium, Lithodes, Hippa (Fig. 1.113B), Cancer, My sis (Fig. 1.113D), Oniscus, Eupagurus (Fig. 1.113A).

Subphylum Uniramia (Latin: unus, one ramus, branch):

1. Body divided into head and trunk. The trunk either bear pairs of walking legs, or it may be differentiated into thorax and abdomen, with the abdominal appen­dages greatly reduced or missing.

3. Head appendages comprise of one pair each of antennae, mandibles and maxil­lae and in some groups a second pair of maxillae. In addition, it has an upper lip or labrum.

4. Head also comprises of lateral ocelli, frequently organised into compound eyes sometimes also with median ocelli.

5. Gut straight lacking digestive diverticula.

6. Tracheae used for respiration.

7. Excretory organs are malpighian tubules. Uniramians consist of more than one mil­lion species distributed between five classes.

Class Chilopoda (centipedes):

1. Members of this class are carnivorous and distributed throughout the world in both temperate and tropical regions, residing in soil and humus, beneath stones, bark and logs.

2. Body elongated and dorsoventrally flattened.

3. Trunk comprising of 15 to more than 181 leg-bearing segments, the last two seg­ments being legless.

4. They attain lengths of up to 27 cm.

5. The first pair of legs (forcipules) are large commonly called poison claws. It termi­nates into a pointed fang, which is the outlet for the duct of a poison gland.

6. The sense organ, ‘Organs of Tomosvary’ is present as a single pair at the base of the antennae.

7. The genital segment of both sexes carries small appendages (gonopods) which help in reproduction.

There are about 3000 species distributed between two subclasses.

1. Adults possess 21 or more pairs of legs.

2. Brooding of egg takes place.

3. Young’s possess all segments on hatch­ing.

Scolopendra (Fig. 1.114A), Theatops, Geophilus, Strigamia

2. Full complement of segments are not possessed by the young’s.

3. Adults possess 15 pairs of legs.

Scutigera (Fig. 1.114B), Lithobius (Fig. 1.114C), Bothropolys.

1. The symphylans are small mainly herbi­vorous myriapods that live in soil and leaf mold in most parts of the world.

2. Body comprises of a head and a long trunk with twelve leg bearing segments and two terminal segments without leg. The last segment bears a pair of long sensory hair (trichobothria).

3. Mouth parts comprise of a pair of mandibles, a pair of long, first maxillae and a second pair of maxillae fused together forming a labium. This is appa­rently similar to that of insects.

4. There are more dorsal tergal plates (15 to 24) than the number of segments. This permit increased flexibility of the body.

5. Presence of a single pair of spiracles that open on the sides of the head.

7. Sexes separate. Parthenogenesis is com­mon. Copulatory behaviour is unusual. The young’s have six or seven pairs of legs.

This class comprises of 160 described species.

Class Diplopoda (millipedes):

1. Diplopods commonly known as mille­pedes (thousand leggers), live beneath leaves, stones, barks, logs and in soil. They are distributed throughout the world.

2. Presence of double trunk segments, referred to as diplosegments, formed from the fusion of two originally separate somites. Each diplosegment bears two pairs of legs.

3. Trunk is composed of leg-less first seg­ment, followed by three segments each with single pair of legs and then from 5 to more than 85 segments (diploseg­ments), each with two pairs of legs, ganglia, heart, ostia etc.

4. The floor of the preoral chamber is formed by a fused pair of maxillae, called the gnathochilarium. Second pair of maxillae is absent.

5. Presence of calcified exoskeleton (the only among uniramians).

6. Repugnatorial glands present in many.

7. Eyes may be totally absent (flat-backed millipedes), or there may be 2 to 80 ocelli arranged about the antennae. Many possess ‘Organs of Tomosvary’.

8. Gonopores are located at the anterior end of the trunk (third trunk segment).

9. Development is anamorphic.

About 10,000 species have been identi­fied and disposed in three subclasses.

Subclass Pencillata (Pselaphognatha):

1. Minute with soft integument bearing tufts and rows of serrated scale-like setae.

2. Trunk bears 13 to 17 pairs of legs.

4. No repugnatorial glands.

Polyxenus, Lophoproctus Subclass Pentazonia

2. Last two pairs of legs modified for clasping.

Subclass Helminthomorpha:

1. Segments are either cylindrical or some­what flattened.

2. At least one pair of legs (gonopods) of the seventh segment in the male modi­fied for sperm transfer.

Julus (Fig. 1.115), Polyzonium, Polydesmus, Orthoporus, Narceus, Thyropygns.

1. Pauropods constitute soft-bodied, grub-­like animals that inhabit leaf mold and soil. They are widespread in both tropi­cal and temperate regions.

2. Body comprises of a head and eleven segmented trunk, nine of which bear a pair of legs. The first and last two seg­ments are legless.

3. The tergal plates present on the dorsal surface of trunk, are large and overlap adjacent segments. Five of them carry a pair of long, laterally placed setae.

4. Head bears a single pair of maxillae. The antennae are biramous, with one parti­tion terminating in a single and the other in two flagella.

5. Head lacks median ocelli but bears the ‘Organs of Tomosvary’.

6. Absence of heart and trachea.

7. Development is anamorphic.

There are approximately 500 described species.

Class Insecta (Hexapoda):

1. Head formed by the fusion of 6 seg­ments typically bears a single pair of antennae and two pairs of maxillae.

2. The trunk in case of insects is subdivided into a three segmented thorax and an abdomen of eleven segments without walking legs.

3. The thoracic region bears three pairs of legs and usually two pairs of wings.

4. The head in addition, also possesses median ocelli as well as lateral ocelli or compound eyes.

5. Foregut is commonly subdivided into an anterior pharynx, an esophagus, a crop and a narrow proventriculus. The proventriculus is variable in structure and function, in different insects depen­ding upon the nature of food taken.

6. Most insects possess a pair of salivary or labial glands.

7. Gas exchange takes place through a system of trachea.

8. The chief excretory organs are malpighian tubules which remain closely asso­ciated with alimentary canal.

9. Gonoducts open at the posterior end of the abdomen.

Insecta or Hexapoda comprises of more than 7,50,000 described species. It is three times larger than all the other animal groups combined. It is divided into two subclasses comprising of 26 orders.

1. Mouth parts sunk into a head pouch.

2. Malpighian tubules and compound eyes are either reduced or totally absent.

3. Absence of wings and this condition is primary.

Isotoma (Springtail) (Fig. 1.117C), Acerentulus (telsontail), Campodea.

1. Mouth parts are not sunk into a pouch.

2. Malpighian tubules are present.

3. Presence of compound eyes.

4. The orders are winged forms, except for two wingless orders Microcoryphia and Thysanura.

5. Presence of an ovipositor derived from the eighth and ninth abdominal seg­ments.

Lepisma (Silver fish) (Fig. 1.117A), Machilis.

Periplaneta, Carausius (stick insect) (Fig. 1.117D), Bombyx, Anopheles, Apis, Formica (ant), Acheta (House cricket) (Fig. 1.117B), Phyllium (leaf insect) (Fig. 1.117E), Tachardia (Lac insect).

Relationship with Annelids:

Arthropods are clearly related to annelids and their relationship is displayed in the following ways:

1. Like annelids, arthropods are segmen­ted.

2. The nervous system of annelids and arthropods are constructed on the same basic plan (ventral nerve cord proceeded by a dorsal anterior brain).

3. Few arthropod’s embryonic develop­ment displays some degree of spiral determinate cleavage (like that of annelids).

4. In the primitive condition, each arthro­pod segment bears a pair of appendages — a condition which is still displayed by the polychaetes.

However, it is still uncertain, whether arthropods have arisen from annelids or both from same common ancestor.

27.2 Features Used to Classify Animals

By the end of this section, you will be able to do the following:

  • Explain the differences in animal body plans that support basic animal classification
  • Compare and contrast the embryonic development of protostomes and deuterostomes

Scientists have developed a classification scheme that categorizes all members of the animal kingdom, although there are exceptions to most “rules” governing animal classification (Figure 27.6). Animals have been traditionally classified according to two characteristics: body plan and developmental pathway. The major feature of the body plan is its symmetry: how the body parts are distributed along the major body axis. Symmetrical animals can be divided into roughly equivalent halves along at least one axis. Developmental characteristics include the number of germ tissue layers formed during development, the origin of the mouth and anus, the presence or absence of an internal body cavity, and other features of embryological development, such as larval types or whether or not periods of growth are interspersed with molting.

Visual Connection

Which of the following statements is false?

  1. Eumetazoans have specialized tissues and parazoans don’t.
  2. Lophotrochozoa and Ecdysozoa are both Bilataria.
  3. Acoela and Cnidaria both possess radial symmetry.
  4. Arthropods are more closely related to nematodes than they are to annelids.

Animal Characterization Based on Body Symmetry

At a very basic level of classification, true animals can be largely divided into three groups based on the type of symmetry of their body plan: radially symmetrical, bilaterally symmetrical, and asymmetrical. Asymmetry is seen in two modern clades, the Parazoa (Figure 27.7a) and Placozoa. (Although we should note that the ancestral fossils of the Parazoa apparently exhibited bilateral symmetry.) One clade, the Cnidaria (Figure 27.7b,c), exhibits radial or biradial symmetry: Ctenophores have rotational symmetry (Figure 27.7e). Bilateral symmetry is seen in the largest of the clades, the Bilateria (Figure 27.7d) however the Echinodermata are bilateral as larvae and metamorphose secondarily into radial adults. All types of symmetry are well suited to meet the unique demands of a particular animal’s lifestyle.

Radial symmetry is the arrangement of body parts around a central axis, as is seen in a bicycle wheel or pie. It results in animals having top and bottom surfaces but no left and right sides, nor front or back. If a radially symmetrical animal is divided in any direction along the oral/aboral axis (the side with a mouth is “oral side,” and the side without a mouth is the “aboral side”), the two halves will be mirror images. This form of symmetry marks the body plans of many animals in the phyla Cnidaria, including jellyfish and adult sea anemones (Figure 27.7b, c). Radial symmetry equips these sea creatures (which may be sedentary or only capable of slow movement or floating) to experience the environment equally from all directions. Bilaterally symmetrical animals, like butterflies (Figure 27.7d) have only a single plane along which the body can be divided into equivalent halves. The Ctenophora (Figure 27.7e), although they look similar to jellyfish, are considered to have rotational symmetry rather than radial or biradial symmetry because division of the body into two halves along the oral/aboral axis divides them into two copies of the same half, with one copy rotated 180 o , rather than two mirror images.

Bilateral symmetry involves the division of the animal through a midsagittal plane, resulting in two superficially mirror images, right and left halves, such as those of a butterfly (Figure 27.7d), crab, or human body. Animals with bilateral symmetry have a “head” and “tail” (anterior vs. posterior), front and back (dorsal vs. ventral), and right and left sides (Figure 27.8). All Eumetazoa except those with secondary radial symmetry are bilaterally symmetrical. The evolution of bilateral symmetry that allowed for the formation of anterior and posterior (head and tail) ends promoted a phenomenon called cephalization, which refers to the collection of an organized nervous system at the animal’s anterior end. In contrast to radial symmetry, which is best suited for stationary or limited-motion lifestyles, bilateral symmetry allows for streamlined and directional motion. In evolutionary terms, this simple form of symmetry promoted active and controlled directional mobility and increased sophistication of resource-seeking and predator-prey relationships.

Animals in the phylum Echinodermata (such as sea stars, sand dollars, and sea urchins) display modified radial symmetry as adults, but as we have noted, their larval stages (such as the bipinnaria) initially exhibit bilateral symmetry until they metamorphose in animals with radial symmetry (this is termed secondary radial symmetry). Echinoderms evolved from bilaterally symmetrical animals thus, they are classified as bilaterally symmetrical.

Link to Learning

Watch this video to see a quick sketch of the different types of body symmetry.

Animal Characterization Based on Features of Embryological Development

Most animal species undergo a separation of tissues into germ layers during embryonic development. Recall that these germ layers are formed during gastrulation, and that each germ layer typically gives rise to specific types of embryonic tissues and organs. Animals develop either two or three embryonic germ layers (Figure 27.9). The animals that display radial, biradial, or rotational symmetry develop two germ layers, an inner layer (endoderm or mesendoderm) and an outer layer (ectoderm). These animals are called diploblasts , and have a nonliving middle layer between the endoderm and ectoderm (although individual cells may be distributed through this middle layer, there is no coherent third layer of tissue). The four clades considered to be diploblastic have different levels of complexity and different developmental pathways, although there is little information about development in Placozoa. More complex animals (usually those with bilateral symmetry) develop three tissue layers: an inner layer (endoderm), an outer layer (ectoderm), and a middle layer (mesoderm). Animals with three tissue layers are called triploblasts .

Visual Connection

Which of the following statements about diploblasts and triploblasts is false?

  1. Animals that display only radial symmetry during their lifespans are diploblasts.
  2. Animals that display bilateral symmetry are triploblasts.
  3. The endoderm gives rise to the lining of the digestive tract and the respiratory tract.
  4. The mesoderm gives rise to the central nervous system.

Each of the three germ layers is programmed to give rise to specific body tissues and organs, although there are variations on these themes. Generally speaking, the endoderm gives rise to the lining of the digestive tract (including the stomach, intestines, liver, and pancreas), as well as to the lining of the trachea, bronchi, and lungs of the respiratory tract, along with a few other structures. The ectoderm develops into the outer epithelial covering of the body surface, the central nervous system, and a few other structures. The mesoderm is the third germ layer it forms between the endoderm and ectoderm in triploblasts. This germ layer gives rise to all specialized muscle tissues (including the cardiac tissues and muscles of the intestines), connective tissues such as the skeleton and blood cells, and most other visceral organs such as the kidneys and the spleen. Diploblastic animals may have cell types that serve multiple functions, such as epitheliomuscular cells, which serve as a covering as well as contractile cells.

Presence or Absence of a Coelom

Further subdivision of animals with three germ layers (triploblasts) results in the separation of animals that may develop an internal body cavity derived from mesoderm, called a coelom , and those that do not. This epithelial cell-lined coelomic cavity, usually filled with fluid, lies between the visceral organs and the body wall. It houses many organs such as the digestive, urinary, and reproductive systems, the heart and lungs, and also contains the major arteries and veins of the circulatory system. In mammals, the body cavity is divided into the thoracic cavity, which houses the heart and lungs, and the abdominal cavity, which houses the digestive organs. In the thoracic cavity further subdivision produces the pleural cavity, which provides space for the lungs to expand during breathing, and the pericardial cavity, which provides room for movements of the heart. The evolution of the coelom is associated with many functional advantages. For example, the coelom provides cushioning and shock absorption for the major organ systems that it encloses. In addition, organs housed within the coelom can grow and move freely, which promotes optimal organ development and placement. The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility, promoting improved animal motility.

Triploblasts that do not develop a coelom are called acoelomates , and their mesoderm region is completely filled with tissue, although they do still have a gut cavity. Examples of acoelomates include animals in the phylum Platyhelminthes, also known as flatworms. Animals with a true coelom are called eucoelomates (or coelomates) (Figure 27.10). In such cases, a true coelom arises entirely within the mesoderm germ layer and is lined by an epithelial membrane. This membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some freedom of movement. Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates. A third group of triploblasts has a slightly different coelom lined partly by mesoderm and partly by endoderm. Although still functionally a coelom, these are considered “false” coeloms, and so we call these animals pseudocoelomates . The phylum Nematoda (roundworms) is an example of a pseudocoelomate. True coelomates can be further characterized based on other features of their early embryological development.

Embryonic Development of the Mouth

Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on differences in the origin of the mouth. When the primitive gut forms, the opening that first connects the gut cavity to the outside of the embryo is called the blastopore. Most animals have openings at both ends of the gut: mouth at one end and anus at the other. One of these openings will develop at or near the site of the blastopore . In Protostomes ("mouth first"), the mouth develops at the blastopore (Figure 27.11). In Deuterostomes ("mouth second"), the mouth develops at the other end of the gut (Figure 27.11) and the anus develops at the site of the blastopore. Protostomes include arthropods, mollusks, and annelids. Deuterostomes include more complex animals such as chordates but also some “simple” animals such as echinoderms. Recent evidence has challenged this simple view of the relationship between the location of the blastopore and the formation of the mouth, however, and the theory remains under debate. Nevertheless, these details of mouth and anus formation reflect general differences in the organization of protostome and deuterostome embryos, which are also expressed in other developmental features.

One of these differences between protostomes and deuterostomes is the method of coelom formation, beginning from the gastrula stage. Since body cavity formation tends to accompany the formation of the mesoderm, the mesoderm of protostomes and deuterostomes forms differently. The coelom of most protostomes is formed through a process called schizocoely . The mesoderm in these organisms is usually the product of specific blastomeres, which migrate into the interior of the embryo and form two clumps of mesodermal tissue. Within each clump, cavities develop and merge to form the hollow opening of the coelom. Deuterostomes differ in that their coelom forms through a process called enterocoely . Here, the mesoderm develops as pouches that are pinched off from the endoderm tissue. These pouches eventually fuse and expand to fill the space between the gut and the body wall, giving rise to the coelom.

Another difference in organization of protostome and deuterostome embryos is expressed during cleavage. Protostomes undergo spiral cleavage , meaning that the cells of one pole of the embryo are rotated, and thus misaligned, with respect to the cells of the opposite pole. This is due to the oblique angle of cleavage relative to the two poles of the embryo. Deuterostomes undergo radial cleavage , where the cleavage axes are either parallel or perpendicular to the polar axis, resulting in the parallel (up-and-down) alignment of the cells between the two poles.

A second distinction between the types of cleavage in protostomes and deuterostomes relates to the fate of the resultant blastomeres (cells produced by cleavage). In addition to spiral cleavage, protostomes also undergo determinate cleavage . This means that even at this early stage, the developmental fate of each embryonic cell is already determined. A given cell does not have the ability to develop into any cell type other than its original destination. Removal of a blastomere from an embryo with determinate cleavage can result in missing structures, and embryos that fail to develop. In contrast, deuterostomes undergo indeterminate cleavage , in which cells are not yet fully committed at this early stage to develop into specific cell types. Removal of individual blastomeres from these embryos does not result in the loss of embryonic structures. In fact, twins (clones) can be produced as a result from blastomeres that have been separated from the original mass of blastomere cells. Unlike protostomes, however, if some blastomeres are damaged during embryogenesis, adjacent cells are able to compensate for the missing cells, and the embryo is not damaged. These cells are referred to as undetermined cells. This characteristic of deuterostomes is reflected in the existence of familiar embryonic stem cells, which have the ability to develop into any cell type until their fate is programmed at a later developmental stage.

Evolution Connection

The Evolution of the Coelom

One of the first steps in the classification of animals is to examine the animal’s body. One structure that is used in classification of animals is the body cavity or coelom. The body cavity develops within the mesoderm, so only triploblastic animals can have body cavities. Therefore body cavities are found only within the Bilateria. In other animal clades, the gut is either close to the body wall or separated from it by a jelly-like material. The body cavity is important for two reasons. Fluid within the body cavity protects the organs from shock and compression. In addition, since in triploblastic embryos, most muscle, connective tissue, and blood vessels develop from mesoderm, these tissues developing within the lining of the body cavity can reinforce the gut and body wall, aid in motility, and efficiently circulate nutrients.

To recap what we have discussed above, animals that do not have a coelom are called acoelomates. The major acoelomate group in the Bilateria is the flatworms, including both free-living and parasitic forms such as tapeworms. In these animals, mesenchyme fills the space between the gut and the body wall. Although two layers of muscle are found just under the epidermis, there is no muscle or other mesodermal tissue around the gut. Flatworms rely on passive diffusion for nutrient transport across their body.

In pseudocoelomates, there is a body cavity between the gut and the body wall, but only the body wall has mesodermal tissue. In these animals, the mesoderm forms, but does not develop cavities within it. Major pseudocoelomate phyla are the rotifers and nematodes. Animals that have a true coelom are called eucoelomates all vertebrates, as well as molluscs, annelids, arthropods, and echinoderms, are eucoelomates. The coelom develops within the mesoderm during embryogenesis. Of the major bilaterian phyla, the molluscs, annelids, and arthropods are schizocoels, in which the mesoderm splits to form the body cavity, while the echinoderms and chordates are enterocoels, in which the mesoderm forms as two or more buds off of the gut. These buds separate from the gut and coalesce to form the body cavity. In the vertebrates, mammals have a subdivided body cavity, with the thoracic cavity separated from the abdominal cavity. The pseudocoelomates may have had eucoelomate ancestors and may have lost their ability to form a complete coelom through genetic mutations. Thus, this step in early embryogenesis—the formation of the coelom—has had a large evolutionary impact on the various species of the animal kingdom.

What kind of arthropod/animal is this? - Biology


23. The Animal Kingdom

23.4. Body Plans

Although animals come in a variety of sizes and shapes, you can see certain evolutionary trends and a few basic body plans.

Symmetrical objects have similar parts that are arranged in a particular pattern. For example, the parts of a daisy flower and a bicycle are arranged symmetrically.

Asymmetry is a condition in which there is no pattern to the individual parts. Asymmetrical body forms are rare and occur only in certain species of sponges, which are the simplest kinds of animals.

Radial symmetry occurs when a body is constructed around a central axis. Any division of the body along this axis results in two similar halves. Although many animals with radial symmetry are capable of movement, they do not always lead with the same portion of the body that is, there is no anterior, or head, end. Starfish and jellyfish are examples of organisms with radial symmetry.

Bilateral symmetry exists when an animal is constructed with equivalent parts on both sides of a plane. Animals with bilateral symmetry have a head and a tail region. There is only one way to divide bilateral animals into two mirrored halves. Animals with bilateral symmetry move head first, and the head typically has sense organs and a mouth. The feature of having an anterior head end is called cephalization (cephal = head). It appears that bilateral symmetry was an important evolutionary development since most animals have bilateral symmetry (figure 23.4).

FIGURE 23.4. Kinds of Symmetry

(a) This sponge has a body that cannot be divided into symmetrical parts and is therefore asymmetrical. (b) In animals such as this with radial symmetry, any cut along the central body axis results in similar halves. (c) In animals with bilateral symmetry, only one one plane results in similar halves.

Animals differ in the number of layers of cells of which they are composed. When we look at the development of embryos we find that the embryos of the simplest animals (sponges) do not form distinct, tissuelike layers. However, jellyfishes and their relatives have embryos that consist of two layers. The ectoderm is the outer layer and the endoderm is the inner layer. Because their embryos are composed of two layers, these animals are said to be diploblastic. In adults, these embryonic cell layers give rise to an outer, protective layer and an inner layer that forms a pouch and is involved in processing food.

All the other major groups of animals have embryos that are triploblastic. Triploblastic animals have three layers of cells in their embryos. Sandwiched between the ectoderm and endoderm is a third layer, the mesoderm. In the adult body, the ectoderm gives rise to the skin or other surface covering, the endoderm gives rise to the lining of the digestive system, and the mesoderm gives rise to muscles, connective tissue, and other organ systems involved in the excretion of waste, the circulation of material, the exchange of gases, and body support (figure 23.5).

FIGURE 23.5. Embryonic Cell Layers

Diploblastic organisms have two embryonic cell layers. The outer ectoderm becomes the epidermis and the inner endoderm becomes the lining of the gut. Triploblastic organisms have three embryonic cell layers: the ectoderm, endoderm, and mesoderm. The mesoderm forms most of the tissues and organs of the body.

A coelom is a fluid-filled body cavity that separates the outer body wall of the organism from the gut and internal organs. The development of a coelom was an important step in animal evolution.

Simple animals, such as jellyfish and flatworms, are acoelomate, which means that they have no space separating their outer surface from their internal organs. However, most animals have some form of a coelom. Because organs such as the gut and heart are not embedded in a mass of cells but are suspended in a space (the coelom), they have a greater freedom of movement than the organs of acoelomate animals. Organs are not loose in the coelom they are held in place by sheets of connective tissue called mesenteries.

Mesenteries also support the blood vessels connecting the various organs.

It is often difficult to visualize the presence of a coelom as a cavity, because the cavity is filled with organs and a small amount of fluid. Perhaps a common example will help.

The coelom in a turkey is the cavity where you stuff the dressing. In the living bird this cavity contains a number of organs, including those of the digestive, excretory, and circulatory systems.

Some animals do not have a true coelom but have a similar space called a pseudocoelom. A pseudocoelom differs from a true coelom in that it is located between the lining of the gut and the outer body wall. In other words, animals with a pseudocoelom do not have muscles around their digestive system. In addition, there are no mesenteries suspending the gut from the outer body wall. Nematode worms and several related groups of animals have a pseudocoelom (figure 23.6).

(a) Acoelomate animals, such as flatworms, have no open space between the gut and outer body layer. (b) Roundworms, commonly found in soil, have a body cavity called a pseudocoelom. It contains some cells, and there are no muscles surrounding the gut. (c) Other animals, including all vertebrates, have a coelom, which is a fluid-filled space that separates internal organs from the outer body wall. In addition, the coelom is lined with connective tissue of mesodermal origin. Organs project into the coelom and are held in place by thin sheets of connective tissue called mesenteries.

Many kinds of bilaterally symmetrical organisms have segmented bodies.

Segmentation is the separation of an animal’s body into a number of recognizable units from its anterior to its posterior end. Segmentation is associated with the specialization of certain parts of the body. Three common groups of animals show segmentation: annelid worms, arthropods, and chordates.

Annelid worms have a series of very similar segments with minor differences between them. Segmentation in arthropods is modified so that several segments are specialized as a head region and more posterior segments are less specialized. Many of the posterior segments have legs and other appendages. Among the arthropods, insects show a great deal of specialization of segments. In chordates, the segmentation is of a different sort but is obvious in the arrangement of muscles and the vertebral column (figure 23.7). Studies of the genes that control development show that all bilaterally symmetrical animals have essentially the same genes controlling their development and how different regions or segments of the body develop (How Science Works 23.1).

Segmentation is associated with the specialization of certain parts of the body. Annelid worms show many segments with little specialization. Arthropods show a highly developed head region, with the more posterior segments less specialized. Chordates show the segmentation of muscles and skeletal structures.

Genes, Development, and Evolution

One of the important discoveries of modern molecular genetics is the remarkable similarity in the kinds of genes found in all organisms. This has important implications for understanding the evolution of organisms. It appears that once a new, valuable gene is created through the process of mutation, it is preserved in evolutionary descendants. One example is a group of genes known as homeotic genes. These genes regulate how an organism's body is formed by helping to define which end of the developing embryo is the head and which is the tail. As the embryo develops and regular body segments form, the homeotic genes also help define what each segment becomes. In insects, one segment might give rise to antennae, while another gives rise to wings or legs. Homeotic genes were first discovered in the fruit fly (Drosophila melanogaster), which has been a favorite species for students of animal genetics for 100 years (see photo). Fruit flies are ideal for genetic studies for several reasons: They are easy and inexpensive to raise in the lab, a new generation can be produced every 10 days, and large numbers of offspring are produced.

It is now known that homeotic genes control the same developmental processes in all organisms that are bilaterially symmetrical (their left side mirrors their right side). This trend is so overwhelming that some scientists have suggested that the presence of one type of homeotic genes, the Hox genes, should be used to define the Animal kingdom.

Essentially the same genes with the same functions can be found in widely different animals, such as fruit flies, earthworms, sea urchins, tapeworms, and humans. This means that the study of fruit flies can be used to discover how the same genes function in humans and other animals. Because homeotic genes are involved in regulating embryonic development and cellular differentiation, these studies can be used to help identify the causes of human embryonic development abnormalities and other diseases like cancer.

A skeleton is the part of an organism that provides structural support. Most animals have a skeleton. It serves as strong scaffolding, to which other organs can be attached. In particular, the skeleton provides places for muscle attachment and, if the skeleton has joints, the muscles can move one part of the skeleton with respect to others. Some aquatic organisms, such as sea anemones and many kinds of worms, are generally supported by the dense medium in which they live and lack well-developed skeletons. However, most aquatic animals have a skeleton. Most terrestrial animals have a strong structure that supports them in the thin medium of the atmosphere.

There are two major types of skeletons: internal skeletons (endoskeletons) and external skeletons (exoskeletons) (figure 23.8). The vertebrates (fish, amphibians, reptiles, birds, mammals), echinoderms (starfish, sea urchins, etc.), and some other groups have internal skeletons. The various organs are attached to and surround the skeleton, which grows in size as the animal grows. Arthropods (crustaceans, spiders, insects, millipedes, centipedes), nematodes, and some other groups have an external skeleton that surrounds all the organs. It is generally hard and has joints. These animals accommodate growth by shedding the old skeleton and producing a new, larger one. This period in the life of an arthropod is dangerous, because for a short period it is without its hard, protective outer layer. Many other animals have structures that have a supportive or protective function (such as clams, snails, and corals) and these are sometimes called skeletons, but they do not have joints.

There are two major types of skeletons endoskeletons and exoskeletons. Endoskeletons are typical of vertebrates and echinoderms, and exoskeletons are typical of arthropods and their relatives.

Some organisms use water as a kind of supportive skeleton. Annelid worms and some other animals have fluid-filled coeloms. Because water is not compressible, but it is movable, compressive forces by muscles can cause the animal’s shape to change. This is similar to what happens with a water-filled balloon compression in one place causes it to bulge out somewhere else.

6. Describe body forms that show asymmetry, radial symmetry, and bilateral symmetry.

7. Give an example of an animal that has a coelom and one with a pseudocoelom.

8. Give an example of an animal with an exoskeleton, and one with an endoskeleton.

9. How does an animal with an exoskeleton grow?

10. How do diploblastic and triploblastic animals differ?

11. What is one advantage of segmentation?

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Cicada Snacks: The Wild (and Tasty) Side of Brood X at the Zoo

They slumber underground for 16 summers, nestled near tree roots, sipping xylem — the nutrient-poor water inside tree tissues. Then, as ground temperatures rise on the 17th summer, they emerge and begin blindly burrowing their way toward the surface, bursting forth to a summer of song, flight and love.

It sounds like a spooky fairytale but in fact, it’s the actual true story of the 17-year Brood X cicadas and for some Zoo animals, the beginning of a tasty bug buffet.

Washington, D.C., Maryland and Virginia have cicadas every summer. Their song is the soundtrack to the region’s warmest days and sultriest nights. We rarely see those cicadas, though, because while they do sing, they are also camouflaged to blend into the trees. Their survival strategy is to hide to avoid being eaten.

Brood X cicadas take a different track. With flashy Tonka-truck colored wings and vivid scarlet eyes, they’re nothing if not showy. Their strategy, rather than hiding, is to exist in such vast numbers that enough survive to mate and lay eggs in new young tree stems. The larvae then hatch and fall to the ground looking for grass or any other plants that offer a place to hide before burrowing down to tree roots, ensuring the next Brood X generation emerges 16 years from now.

Magic Cicadas

Three species of cicadas make up Brood X 17-year cicadas, all in the genus Magicicada. The genus name actually means “many”— a reference to their overwhelming numbers — but for entomologist and Amazonia exhibit keeper Donna Stockton at the Smithsonian’s National Zoo and Conservation Biology Institute, it might as well mean what it sounds like: magical.

“I could talk about cicadas all day,” Stockton enthuses. “The 17-year emergence is such an exciting time because it gives people a chance to learn about these amazing insects that otherwise they wouldn’t see, because the cicadas we have every year are camouflaged.”

Their coloring is striking, but what really sets the Brood X cicadas apart, aside from their sheer numbers, are their songs. Specifically, the volume of those songs.

“You think our annual cicadas are loud when you hear their little chirping, wait until you hear the 17-year cicadas,” Stockton said. “Their calls can get really loud — almost like an airplane flying over. Each species has its own call, but they’re all loud.”

People all over the Washington, D.C. region will be able to hear the cicadas, and some lucky, observant ones may also be able to spot the “chimneys” cicadas often build up from the ground. Similar to a crayfish turret, the chimneys are most visible on bare ground without too many leafy trees and can reach heights of up to a foot.

The cicadas crawl out of the ground as the ground temperatures reach 64 or 65 degrees Fahrenheit. They molt a number of times underground, shedding their exoskeletons as they grow larger. By the time they ascend their chimneys and emerge from the soil, they’re brown and crunchy nymphs. They molt one more time before they transform into their final winged stage. Then the males start singing to attract females and mate. The female cicadas lay eggs, and both parents die. Their life cycle is over for another 17 years.

Cicadas at the Zoo

Given that it has been 17 years since their last emergence, zoo keepers and scientists aren’t entirely sure what to expect from Brood X. Though, given the size of the Zoo and the number of trees on it, they’re expecting a fair showing of cicadas.

“There are stories from last time of keepers using shovels and brooms to sweep cicada larvae off Olmsted,” said Mike Maslanka, head of nutrition science, referring to the main pathway through the Zoo named for famous landscape designer Frederick Law Olmsted. “Looking at the map, we do expect more of an impact at the Zoo in Washington, D.C., than out at the Conservation Biology Institute in Front Royal, Virginia.”

When the cicadas emerge from the ground, they are typically as plump, soft and well-fed as they’re going to get, and a number of animals find them irresistible. Maslanka notes that all the bears — sloth bears, Andean bears, and the giant pandas — will likely be interested in the cicadas, along with the maned wolves and otters.

Sloth bears evolved to eat insects. The giant pandas and Andean bears, usually herbivores, are not likely to turn down such tasty plump larvae should they emerge in the panda yard. Maned wolves subsist mainly on small animals and insects, so they are likely to be thrilled by the smorgasbord erupting in their yards. And otters, especially the nimble-pawed Asian small-clawed otters, will delight in digging up and consuming the tasty treats.

The cicadas are safe for the Zoo animals to eat. The only concern Maslanka has is making sure that none of the animals eat too many of them. They know the cicadas are high in protein, but they are insects and, when in their winged form, they can be harder to digest than other, softer, foods. The nutritionists are discussing possibly collecting and freezing some of the larva to study or save for animal enrichment later.

Watch the video: Characteristics of Arthropods (May 2022).