sea urchin


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sea urchin,

spherical-shaped echinoderm with movable spines covering the body. The body wall is a firm, globose shell, or test, made of fused skeletal plates and marked by regularly arranged tubercles to which the movable spines are attached. Five rows of the skeletal plates are pierced by pores for the tube feet of the water-vascular system; these are typical of echinoderms and are used for locomotion. The mouth is centered on the lower side of the body and in many species is surrounded by a whorl of gills. A complex jaw and tooth apparatus in the mouth, known as Aristotle's lantern, is used to fragment food. Long, sharp spines are used for protection, and in some species are poisonous. The spines are also used as levers, aiding the tube feet in locomotion and, along with the teeth, are used by some species to dig burrows in hard rock. Sea urchins feed on all kinds of plant and animal material; some eat sand or mud, digesting out organic material that is present. Entirely marine, they occur in all seas and at all depths but prefer shallower waters and rocky bottoms. Arbacia and Strongylocentrotus are the most familiar American genera; one species of the latter, the red sea urchin (S. franciscanus) of the Pacific coast, is estimated to live for 200 years or more. Eggs and sperm are shed into the sea. After fertilization, a characteristic, free-swimming larva, called the pluteus larva, develops; it undergoes a profound metamorphosis to assume the adult form. Sea urchins have some economic significance. The roe is considered a delicacy, especially in Mediterranean regions and Japan, and burrowing species may damage sea walls. Sea urchins also are used in embryological studies. Sea urchins are classified in the phylum EchinodermataEchinodermata
[Gr.,=spiny skin], phylum of exclusively marine bottom-dwelling invertebrates having external skeletons of calcareous plates just beneath the skin. The plates may be solidly fused together, as in sea urchins, loosely articulated to facilitate movement, as in sea
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, class Echinoidea, subclass Regularia.

Sea Urchin

 

any one invertebrate of the class Echinoidea of the phylum Echinodermata. The body, measuring as much as 30 cm, is covered with rows of skeletal plates that form a shell and bear movable spines and pedicellariae. Sea urchins of the subclass Regularia have a mouth with a masticatory apparatus (Aristotle’s lantern) for scraping algae off rocks. Those of the subclass Irregularia, who feed on detritus, have no masticatory apparatus. Sea urchins are benthic crawling or burrowing animals, moving by means of tube feet and spines. They are dioecious. A stage in their development is the plankton larva, or the echinopluteus; some are viviparous. More than 800 species of sea urchins are extant; there are about 40 species in seas of the USSR. They are widespread in oceans and seas with normal salinity at depths up to 7 km. Some are valuable commercially since the eggs are edible. Fossils of sea urchins have been found in Ordovician deposits.

REFERENCES

Zhiznzhivotnykh, vol. 2. Moscow, 1968.
Hyman, L. H. The Invertebrates, vol. 4. New York-London, 1955.

sea urchin

[′sē ‚ər·chən]
(invertebrate zoology)
A marine echinoderm of the class Echinoidea; the soft internal organs are enclosed in and protected by a test or shell consisting of a number of close-fitting plates beneath the skin.

sea urchin

any echinoderm of the class Echinoidea, such as Echinus esculentus (edible sea urchin), typically having a globular body enclosed in a rigid spiny test and occurring in shallow marine waters
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