The phylum Echinodermata consists of about 7,000 species distributed in five classes: Crinoidea, Asteroidea, Ophiuroidea, Echinoidea and Holothuroidea.
The group name is derived from two Greek words: echinos, which means thorn, and derma, which means skin, and refers to projections in the form of thorns or tubers present on the surface of the body.
All phylum representatives are free-living, with rare commensal species. Many are adapted to attach to rocky substrates, while others live on muddy, sandy substrates, submerged wood or epibiosis. Although the vast majority of species are marine, some tolerate brackish water. They can be found in all oceans, latitudes and depths, from intertidal to abyssal regions, being more abundant in the tropical region than in polar waters.
The echinoderms comprise five classes, listed in the following table:
|Echinoidea||quasi-spherical body with large movable spines (hedgehogs) or flat with short fixed spines (sea crackers)||sea urchins,|
sea crackers or biscuits.
|Asteroid||large body with five or more arms; small fixed spines||starfish|
|Crinoid||starry body with branched arms; without thorns||sea lilies|
|Holothuroidea||cylindrical and thornless body; absence of arms||sea cucumbers|
|Ophiuroidea||starry body, with well delimited central disc; short or long spines on the arms||sea snakes|
It is complete, that is, after the mouth follow the esophagus, stomach, intestines and anus. In the sea urchins the oral (lower) face has the mouth, a scraper structure with five teeth, called Aristotle's lantern. This structure is responsible for obtaining the food and corroding the rock that the animal settles on. On the opposite side, the aboral (upper), is the anus.
Sea urchin anatomy
Starfish feed mainly on small mollusks such as shellfish. With its small feet, the starfish forces open the oyster shells, then turns its own stomach inside out and throws a digestive juice into the shells. Then just swallow the mass, that is, the body of the already digested clam. This is therefore a cardiopulmonary digestion.
It is absent or rudimentary, and materials are distributed through the cellomatic cavity. Excretion is done directly through the water that occupies the ambulatory system, there being no other specialized excretory structure.