Sea urchins
   Classis Echinoidea Leske, 1778

Unarmed globular sea urchin
Intermediate globular sea urchin
Beautiful globular sea urchin
Uncommon Scaphechinus
Gray Scaphechinus
Common flat sea urchin
Heart-shaped sea urchin

Sea urchinsSea urchins are globular echinoderms characterized by secondary simplificationloss of arms. Sea urchins have a rigid shell comprising interconnected skeletal plates bearing pedicillaries and mobile needles by means of which certain urchins move along the substrate or dig into the ground. Like starfish, urchins also move with the aid of ambulacral lets emanating through pores in ambulacral plates on the abdominal side of the shell. The mouth is situated on the lower (abdominal) side of the body, as a rule in the center, and is supplied with a characteristic gnawing  oral apparatus - Aristotles lantern--, ideally adapted for grinding algae and little-moving animals. Indeed, this amazing apparatus, representing a system of mobile teeth and muscles, was first described by Aristotles himself. One time, the great naturalist, forced to flee from Athens to live on Greek islands, where he used to stroll along the beach during low tides, collecting and dissecting sea animals. Unlike starfish, certain urchins had lost their regular radial form in body structure to become bilaterally symmetric: the anus situated on the back had shifted backward, and the mouth on the abdominal side conversely forward. Such urchins, unlike regular radial ones, are called irregular sea urchins. This happened in connection with their transition to a digging lifestyle. Digging urchins began to feed on detritus; as a result, their Aristotles lantern had also reduced, and they began to seize their food with specialized mucous-ciliate ambulacral legs.
    As a rule, development in urchins is indirect, i.e. there is first a floating plankton larva, which as it grows and develops changes in form to pass several stages that have special names: dipleurula and three pluteus stages, the last of which by way of catastrophic metamorphosis transforms the pluteus into a tiny young urchin. In accord with their specific structure, urchin pluteuses are called echinopluteuses and ophiopluteuses. In rare cases, they take care of their offspring.
    The recent fauna of sea urchin is represented by 950 species. In Southern Primorye, we know of eight urchin species: three globular or regular, and five irregular urchins (four flat or disk-shaped and one cordiform).