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Reading: Chordates - Biology

Reading: Chordates - Biology


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Chordates

The diagram below shows evolutionary relationships among the major clades of chordates but avoids classification into subphylum and classes. For example, the close relationship between Myxini and the rest of the vertebrates can be seen even though there is uncertainty in the classification of Myxini.

Chordates exhibit bilateral symmetry.

Chordates have the following characteristics at some point in their life history:

  1. a dorsal, hollow nerve cord.
  2. a dorsal supporting rod called a notochord. This is replaced by a vertebral column in vertebrates.
  3. pharyngeal clefts (pouches). These develop into openings to the exterior (gill slits) in some chordates. Gill slits functioned as a mechanism for filter-feeding in primitive vertebrates. The gills of fish function in gas exchange.
  4. a postanal tail. In most other kinds of animals, the digestive tract extends the entire length of the animal.

Subphylum Urochordata (Tunicates)

Larvae

The larvae of tunicates resemble the ancestral chordate. It has chordate characteristics and looks like a tadpole. The free-swimming larva develops into a sessile, filter-feeding adult.

Adult

The adult has a thick-walled body sac and an incurrent siphon and an excurrent siphon. Gill slits are the only chordate feature retained by the adult form. In some tunicates, the adult form may have been lost. These animals retained the larval form as adults.

Subphylum Cephalochordata (Lancelets)

Examine a slide of a lancelet using a dissecting microscope.

Although this animal is not a vertebrate, it has all three of the chordate characteristics. Identify the notochord, nerve chord, and pharyngeal gill slits.

Notice the segmented pattern of the muscles, also a chordate characteristic.

When feeding, water enters the mouth and moves into the pharynx, a chamber posterior to the mouth. The gill slits are openings in the wall of the pharynx and function to allow water to pass out of the pharynx while filtering particles out of the water. The particles move into the gut for digestion. After passing through the gill slits, water exits via the atriopore.

Examine a preserved lancelet and observe the segmented pattern of muscles and the atriopore located on the ventral surface posterior to the pharynx.

Craniates

Craniates are chordates with a head. The head contains sensory organs because it is in front of the animal as it moves through the environment.

The evolution of a head with well-developed sensory organs and a corresponding large brain enabled these animals to be active and to feed by predation or other means that required an active animal.

Hagfishes (Class Myxini)

Hagfishes have a cartilaginous skull but do not have jaws. They do not have vertebrae; their notochord provides the support necessary for their muscles to produce movement.

Subphylum Vertebrata (Vertebrates)

The notochord of vertebrates is generally replaced by a vertebral column composed of numerous small bones called vertebrae that are joined together to form a flexible supporting structure. In most vertebrates, the vertebrae surround the spinal cord.

The vertebral column allows the body to flex and provides attachment sites for muscles. In addition, it surrounds and protects the nerve cord.

They exhibit extreme cephalization and possess complex sense organs (ex: eyes, ears).

Lampreys (Class: Petromyzontida)

Lampreys do not have jaws. Most species of lampreys are parasites. They attach to host fish and feed on the blood of the host.

The larvae are filter feeders that live in freshwater streams. As they mature, they move downstream to the ocean (or lakes) and begin a parasitic life style.

The skeleton is cartilage and the notocord persists in the adult.

Gnathostomes

Gnathostomes are vertebrates with jaws. The evolution of jaws promoted the switch from filter-feeding to predation and thus promoted an active life style. Jaws evolved from the forward gill supports in fish.

The appearance of jaws transformed the worlds ecology due to improved predation and herbivory. As a result, complex food chains evolved.

Filter feeding (gill slits) became less important with the evolution of jaws because jaws allowed the animal to chew larger food items and to capture prey. Gills became more important in gas exchange.

Aquatic gnathostomes have a lateral line system. This system is composed of a line of sensory organs on each side of the body. It is able to detect vibrations in the water.

Cartilaginous Fish (Class: Chondrichthyes)

Chondrichthyes include the sharks, and rays. The cartilage skeleton of sharks is partially hardened with calcium. This type of skeleton is strong and is more flexible and lighter than a bony skeleton. The bodies of cartilaginous fish are covered with small toothlike scales. Their teeth are larger versions of these scales.

They do not have a swim bladder but the oil-storing capacity of their livers improves their buoyancy. The shape of the head and caudal fin also lift the animal as it swims. Sharks must swim to keep from sinking.

Some sharks are fast-swimming predators; others are filter feeders.

Reproduction

Fertilization is internal. The pelvic fin is used to transfer sperm to the female.

Some species are oviparous—they lay eggs that hatch outside the mother’s body; some are ovoviviparous—the eggs are retained within the body and young are born alive; and a few are viviparous—they receive some nourishment via a placenta that develops from the yolk sac of the egg.

The reproductive, digestive and excretory system exit the body through a common opening called a cloaca.

Osteichthyans

Osteichthyans have an ossified (bony) skeleton. The skeleton is hardened with calcium phosphate.

Characteristics of Aquatic Osteichthyans (Bony Fish)

The gills are covered by an operculum so that the gill chamber is enclosed and protected. A swim bladder and is used for buoyancy. It evolved from lungs. The gas content, and thus buoyancy, can be regulated by transfer to and from the blood.

Bony fish have broad, flat scales. The skin contains mucous secreting glands that reduce friction as the fish swims through the water. A lateral line system detects vibrations in the water. Most species are oviparous. Bony fish are the largest group of vertebrates. Approximately 49,000 species have been identified.

Ray-finned Fishs (Class Actinopterygii)

The fins are supported by spinelike rays. In ray-finned fishes, the lungs gave rise to the swim bladder which gives the fish buoyancy.

Lobe-fins

Lobe-finned fish have fins located on fleshy appendages. This group includes coelacanths (class Actinistia), Lungfishes (class Dipnoi) and tetrapods.

Coelacanths were thought to have been extinct for 75 million years until one was captured in 1938 off the southeastern coast of Africa. The first vertebrate animals to develop lungs were fish. The lungs developed from a sac-like pocket of tissue that formed in the pharynx.

Lungfishes are a group of lobe-finned fish that inhabit stagnant fresh water ponds that dry up. Their lungs allow them to gulp oxygen from the air when it has been depleted from the water. Their lobe fins enable them to walk under water.

The ancestors of lungfish gave rise to amphibians.


LICENSES AND ATTRIBUTIONS

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  • Chordates, Biology 102. Authored by: Michael J. Gregory, Ph.D..

    Reading: Chordates - Biology

    Access the page “Reading: Echinoderms.”

    Questions

    1. Skip viewing the slide of the different developmental phases of the sea stars
    2. Dissect the starfish following the directions on the website. Remember the oral end (with mouth) is actually on the underside of the starfish.
      1. On the oral side make sure you find the mouth.
      2. Also on the oral side in the center region of each leg look for the tube feet. Tube feet are used for locomotion powered by the water vascular system. How many rows of tube feet does your starfish have?
      3. Try to differentiate between the spines and the skin gills. The spines are longer are used for protection. The skin gills are smaller and used for gas exchange.
      4. Find the sieve plate/madreporite on the aboral side. This is the water entrance point for the water vascular system used for movement.
      5. The starfish has plates located underneath the skin for protection and support. What material comprises these plates?
      6. The starfish has a two part stomach, the upper pyloric stomach and the lower cardiac stomach. Can you differentiate between the two stomachs on your specimen?
      7. In the starfish arms you should find both digestive glands and gonads. The digestive glands are brown and typically on top of the off white gonads. Make sure you can identify both structures.

      Life Cycle

      The young of some arthropods look like the adults. The change from young to adult that these species undergo is called incomplete metamorphosis.

      In many species, the egg hatches to produce a larva (pl. larvae) that does not look like the adult. At some point in the maturation process, the larva will produce a pupa (pl. pupae). In this phase its tissues will become reorganized into the adult form. This type of development is called complete metamorphosis.

      Observe a culture of fruit flies (Drosophila) and observe the eggs, larvae, pupae, and adults.


      Examples of Chordates

      Lampreys

      The lamprey is a chordate that belongs to the subphylum vertebrata. It is a jawless fish that lives as a filter-feeder in its many larval years, then transforms into a parasitic adult that has an oral disk filled with teeth that it uses to latch onto other fish. Note, however, that some adults don’t feed but rather live off reserves that they obtained as larvae. The adult has external gill slits for breathing, a cartilaginous skeleton, a nerve cord and a notochord. Larval lampreys use a mucus-secreting organ for trapping food particles in their pharynx, and have a post-anal tail. A lamprey larva is shown below.

      Sea Squirts

      The sea squirt is an Urochordate, or Tunicate. As we can see in the picture below, its body is barrel-shaped and is attached to the substrate. The larvae, which are tadpole-like, possess a notochord, a dorsal nerve, pharyngeal slits, and a post-anal tail. The adults are filter-feeders, having two openings, called the siphons. The siphons are used in filter-feeding by drawing water into one side and straining the phytoplankton, which is the sea squirt’s food source, then pumping out the filtered water through the other siphon.

      These two examples are of relatively simple and small organisms, but remember that the phylum Chordata comprises a wide variety of animals. It’s amazing that such small animals can share a significant amount of characteristics with birds, mammals and plenty of other animals.


      Reading: Chordates - Biology

      30-1 The Chordates Reading Guide . Name ______________________________

      1. Members of phylum Chordata are called _________________________________________.
      2. A chordate is an animal that has a dorsal, hollow __________________________________ a notochord ______________________ pouches and a _______________________
      3. The hollow nerve cord runs along the < FRONT or BACK >of the body. (circle one)
      4. Most chordates have a notochord as adults only. < TRUE or FALSE >
      5. Pharyngeal pouches may develop into gills in chordates such as _______________ and _____________________________________.

      Match the description on the left with its chordate characteristic

      a. Notochord. b. Tail. c. Pharyngeal pouch. d. Hollow nerve cord

      _____ 6. Connects nerves to internal organs, muscles & sense organs
      ____ 7. Long supporting rod located just below nerve cord
      _____ 8. Paired structure in throat region
      _____ 9. Contains bone and muscle


      11. About ___________% of all chordates are placed in subphylum ________________________
      12. The dorsal, hollow nerve cord is called the ________________________________in vertebrates
      13. The notochord is replaced with _____________________ in vertebrates and is made up of individual segments called ____________________.
      14. A vertebrate's backbone is part of an ________________________________.
      15. Look at figure 30 - 2 page 768 Ò To which other vertebrate group are birds most closely related?_____________________________________________________
      16. The two groups of nonvertebrate chordates are ______________________________ and _________________________________________________
      17. Tunicates belong to subphylum _______________________________________.
      18. Most tunicates are commonly known as __________________________________.
      19. Lanceletes belong to subphylum _________________________________________.
      20. What makes an adult lancelet different from the adult tunicate? ________________________
      21. What does a lancelet use its pharynx for? ____________________________________
      22. Lancelets have a ________________________ circulatory system.
      23. Lancelets have a true heart. < TRUE or FALSE >


      Invertebrate Chordates

      Chordata also contains two clades of invertebrates: Urochordata and Cephalochordata. Members of these groups also possess the four distinctive features of chordates at some point during their development.

      Urochordata

      Members of Urochordata are also known as tunicates (Figure 3). The name tunicate derives from the cellulose-like carbohydrate material, called the tunic, which covers the outer body of tunicates. Although adult tunicates are classified as chordates, they do not have a notochord, a dorsal hollow nerve cord, or a post-anal tail, although they do have pharyngeal slits. The larval form, however, possesses all four structures. Most tunicates are hermaphrodites. Tunicate larvae hatch from eggs inside the adult tunicate’s body. After hatching, a tunicate larva swims for a few days until it finds a suitable surface on which it can attach, usually in a dark or shaded location. It then attaches via the head to the surface and undergoes metamorphosis into the adult form, at which point the notochord, nerve cord, and tail disappear.

      Figure 3. (a) This photograph shows a colony of the tunicate Botrylloides violaceus. (b) The larval stage of the tunicate possesses all of the features characteristic of chordates: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. (c) In the adult stage, the notochord, nerve cord, and tail disappear. (credit: modification of work by Dann Blackwood, USGS)

      Most tunicates live a sessile existence on the ocean floor and are suspension feeders. The primary foods of tunicates are plankton and detritus. Seawater enters the tunicate’s body through its incurrent siphon. Suspended material is filtered out of this water by a mucous net (pharyngeal slits) and is passed into the intestine via the action of cilia. The anus empties into the excurrent siphon, which expels wastes and water. Tunicates are found in shallow ocean waters around the world.

      Cephalochordata

      Members of Cephalochordata possess a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail in the adult stage (Figure 4). The notochord extends into the head, which gives the subphylum its name. Extinct members of this subphylum include Pikaia, which is the oldest known cephalochordate. Pikaia fossils were recovered from the Burgess shales of Canada and dated to the middle of the Cambrian age, making them more than 500 million years old.

      Extant members of Cephalochordata are the lancelets, named for their blade-like shape. Lancelets are only a few centimeters long and are usually found buried in sand at the bottom of warm temperate and tropical seas. Like tunicates, they are suspension feeders.

      Figure 4. The lancelet, like all cephalochordates, has a head. Adult lancelets retain the four key features of chordates: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Water from the mouth enters the pharyngeal slits, which filter out food particles. The filtered water then collects in the atrium and exits through the atriopore.


      Section Summary

      The five characteristic features of chordates present during some time of their life cycles are a notochord, a dorsal hollow tubular nerve cord, pharyngeal slits, endostyle/thyroid gland, and a post-anal tail. Chordata contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets), together with the vertebrates in the Vertebrata/Craniata. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms. Most tunicates live on the ocean floor and are suspension feeders. Which of the two invertebrate chordate clades is more closely related to the vertebrates continues to be debated. Vertebrata is named for the vertebral column, which is a feature of almost all members of this clade. The name Craniata (organisms with a cranium) is considered to be synonymous with Vertebrata.


      Cephalochordata

      Members of Cephalochordata possess a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail in the adult stage (Figure 2). The notochord extends into the head, which gives the subphylum its name. Extinct members of this subphylum include Pikaia, which is the oldest known cephalochordate. Pikaia fossils were recovered from the Burgess shales of Canada and dated to the middle of the Cambrian age, making them more than 500 million years old.

      Extant members of Cephalochordata are the lancelets, named for their blade-like shape. Lancelets are only a few centimeters long and are usually found buried in sand at the bottom of warm temperate and tropical seas. Like tunicates, they are suspension feeders.

      Figure 2. The lancelet, like all cephalochordates, has a head. Adult lancelets retain the four key features of chordates: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Water from the mouth enters the pharyngeal slits, which filter out food particles. The filtered water then collects in the atrium and exits through the atriopore.



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