Sea Urchin Sea urchins are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone — from the intertidal seashore down to 5,000 meters (16,000 ft; 2,700 fathoms).
The spherical, hard shells (tests) of sea urchins are round and spiny, ranging in diameter from 3 to 10 cm (1 to 4 in). The shell is very strong because it consists of plates covered by calcium carbonate. The test is typically round or slightly ovate and may be speckled with unicellular algae. The part of the "test" that is most often visible above the surface of the sea is commonly called the "umbo," and is not a significant respiratory structure.
Spines are strong, sharp, calcareous, hollow, and come in a variety of shapes and sizes. They are usually long and slender but may be shorter or thicker and stouter. The spines form on the surface of the sea urchin in five longitudinal rows: one ambulacral row which lies just under the peristome (teeth-rim), two interambulacral rows which lie just above the peristome, and two sursummedial rows.
The mouth is a slit-like oral opening at the top of the upper surface of the animal where it can be partly everted. The anus is located on the lower underside a short way inside a small genital opening from which urine and reproductive cells are discharged.
The internal organs of an echinoid are contained within the calcitic test, usually fused to it by skeletal plates which extend inward from the body wall. The anus is near the upper surface. A complex system of water vessels and tube feet draws in seawater along four tunnels leading to the respiratory trees on either side of the mouth.
The anus is usually located on the upper, oral side of the animal. Two gonads are present in both male and female adults, lying close to the kidneys (which lie next to the pericardial cavity). The forms are taken by sea urchins vary with their mode of life; those that crawl over hard surfaces are usually less symmetrical than those that live on soft sea bottoms.
The mouth of sea urchins opens into an internal, madreporite canal system. The madreporite is a network of canals located in the test which are connected to numerous lateral pores on the body wall. This enables sea urchins to rapidly obtain oxygenated water and helps protect them from changes in salinity.