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Formed by an annular nerve around the pharynx and radial nerves, it is rudimentary and has no cephalization.
There are tactile and olfactory cells all over the body surface. Starfish have photoreceptor cells at the ends of their arms.
It occurs by diffusion between seawater and the one occupying the ambulatory system. In sea cucumbers, there are a series of filaments around the mouth, through which passes the cellomatic liquid, which function as gills. There are no oxygen carrying pigments.
Sea urchins have dermal gills, similar to the perioral gills of the sea cucumbers and also occupied by cellomatic liquid. Between the dermal gills and the numerous thorns, the sea urchins have appendages called pedicelaria, which have tweezers at the ends and are used to clean up debris deposited on the body. In some species, these pedicelaria inoculate poison.
O endoskeleton It consists of calcareous plaques distributed in five alternate ambulacral zones with five interambulacral zones.
Ambulacral areas have numerous holes through which the ambulacral feet project, structures related to locomotion. On the dorsal surface of the skeleton there is a central plate or disc (where the anus opens), surrounded by five plates, each with a genital orifice. In addition to the genital orifice, one of these plaques has numerous pores connected to the ambulacral system: it is the mother-to-plaque.
Seated on the plates are the thorns, endowed with mobility thanks to the muscles present in its base. Among the spines, small structures with tweezers, the pedicelaria, consisting of two or three articles, with functions of defense and cleaning the body surface.
The sexes are separated and fertilization is external. Gametes are released into the water, where fertilization occurs. After embryonic development, a larva is formed that has bilateral symmetry.
Metamorphosing, the larva gives rise to the adult of pentarradial symmetry, that is, many of the structures and organs that participate in the organization of these individuals appear in numbers of 5 or multiple of 5: 5 teeth around the mouth, 5 ovaries, 5 zones. outpatients, 5 radial nerves, etc.
It is interesting to remember that the echinoderms, along with the chordates, are deuterostomies, that is, the blastopore, during embryonic development, differ in anus and the mouth appears at the opposite end from a new orifice.
Hedgehog and the cachoidal endoskeleton