In the animal kingdom, echinoderms are animals with radiated symmetry and prickly skin. In fact, the word echinoderm comes from the Greek and literally means prickly skin.
Most of these animals are covered with hard limestone slabs, bumps and thorns. This protective element is very important for these animals that live in the aquatic environment and are nailed to the bottom of the sea, as they move slowly, are defenseless and are subject to possible attacks.
The best known of the echinoderms is the starfish. Your body structure is complex and has five parts. Its body extends radially from a central region and has no head. Each arm is connected to the central disc. Calcareous spines are projected from a hard layer of the inner skeleton , but are generally not attached to it. These thorns and plates embedded in its flesh provide the animal with a kind of protective armor and a supporting skeleton. The skeleton of a starfish is very flexible and can be appreciated when standing up.
The breath is conducted through many epidermal gills are distributed among the thorns, since oxygen is propagated directly into the body cavity which is full of fluid. With small tweezers that are on the thorns, they can grab small animals.
In relation to its locomotion, it depends on a hydraulic system : the ambulatory apparatus. The digestive and reproductive organs of the starfish extend through the interior of the arms, and at the end of each arm is a small tentacle.
Its main food is the clam, an animal slower than it. To open the shell of a clam, the suction cups of its feet are used, which are fixed on the shells, while the starfish struggles to separate them.
Starfish comprise only one class of echinoderms (species diversity depends on the number of arms and can reach a total of 21). The loss of the arms of the echinoderms is common, but they can regenerate in a short time. The most primitive class of these animals are the crinoids and another class are the holoturoids.
Despite changes in their outward appearance, they all have the same basic bodily organization . Thus, starfish, sea urchins and ophiroids are part of the same animal group.