We call echinoderms the animals of about 5500 species of medium sizes, never too big or too small and that are exclusively marine. As an example we can mention the starfish, the sea urchin and the sea cucumber.
Among the main characteristics of echinoderms, we can mention the presence of a hydrovascular system consisting of vessels where water circulates; formations on the body surface triggered by muscles that have jaws; limestone endo-skeleton, that is, an internal skeleton formed by limestone plates covered by the epidermis of mesodermal origin – in some species, these limestone plates emit spines.
They have a complex system of blades, channels and valves known as the aquifer system – or even an ambulatory system, from the Latin ambulare which means walking -, whose functions involve locomotion, breathing, circulation, excretion and perception of the animal. The animals are triblastic, coelomated, deuterostomic and have radial symmetry as adults.
Their locomotion occurs through the ambulacral feet that move due to the variation of the pressure of the accumulated liquid in their muscular walls and ampoules. This variation determines the expansion and retraction of the feet, making possible their displacement: the pressure, when higher, makes them rigid, and when it decreases, it causes them to soften, thus allowing movement.
The echinoderm digestive system has only the mouth, stomach, intestines and anus, and the stomach is present only in carnivorous echinoderms. The animals of this phylum can feed on algae or small animals – as an example of carnivores, we can mention the starfish, which feeds mainly on small mollusks. The starfish feeds through its small feet that force the opening of oyster shells, for example, and turns its stomach inside out, casting its digestive juice over its prey. Then it swallows the body already digested – featuring an extracorporeal digestion. In the case of algae feeders, such as the sea urchin, the mouthpiece – called Aristotle’s lantern – has a kind of dentition through which the algae trapped in the rocks scrape.
The ambulatory system is responsible for eliminating excreta from the body of echinoderms. Through the system, water circulates that helps in the elimination.
The gas exchange of the echinoderms is carried out by means of tiny gills that are located close to the mouth, in addition to the entire length of the ambulatory feet – through the water that passes there -.
Reproduction is sexual, that is, with the participation of gametes – generated by separate sexes – and an external fertilization that takes place in water. With indirect development, the larvae become young animals with their own shape.
Classes and subclasses
- Stelleroid class: itis the class that includes the individuals that have bodies with arms. Among its subclasses are Asteroidea – starfish – which normally have five arms, are carnivorous and scavengers; and ophiuroidea, which is represented by sea snakes. These are scrapers and scavengers, and resemble the starfish, but with longer arms.
- Holothuroid class:It is represented by sea cucumbers – soft and elongated body with no carapace -. Usually they live buried and are commensal and parasitic.
- Echinoid class:with a rounded body, without arms and with mobile and slender spines, this class is represented by beach crackers and sea urchins. While beach biscuits feed on organic particles, hedgehogs, moreover, feed on marine plants.
- Crinoid class:with branched and shiny arms, animals of this class are represented by sea lilies. They feed on microscopic plankton and debris, and live in deeper places.