Sea urchins

Sea urchins or urchins  called sea hedgehogs,are small, spiny, globular animals that, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. About 950 species of echinoids inhabit all oceans from the intertidal to 5000 m deep. The shell, or "test", of sea urchins is round and spiny, typically from 3 to 10 cm (1.2 to 3.9 in) across. Common colors include black and dull shades of green, olive, brown, purple, blue, and red. Sea urchins move slowly, and feed on mostly algae. Sea otters, starfish, wolf eels, triggerfish, and other predators hunt and feed on sea urchins. Their roe is a delicacy in many cuisines. The name "urchin" is an old name for the round spiny hedgehogs, which sea urchins resemble.

Sea urchins are members of the phylum Echinodermata, which also includes sea stars, sea cucumbers, brittle stars, and crinoids. Like other echinoderms, they have five-fold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive "tube feet". The symmetry is not obvious in the living animal, but is easily visible in the dried test. Echinodermate means "spiny skin" in Greek. Like other echinoderms, sea urchins are bilaterans. Their early larvae have bilateral symmetry, but they develop fivefold symmetry as they mature. This is most apparent in the "regular" sea urchins, which have roughly spherical bodies, with five equally sized parts radiating out from their central axes.The lower half of a sea urchin's body is referred to as the oral surface, because it contains the mouth, while the upper half is the aboral surface. The internal organs are enclosed in a hard test or shell composed of fused plates of calcium carbonate covered by a thin dermis and epidermis.

Sea urchins' tube feet arise from the five ambulacral grooves. Tube feet are moved by a water vascular system, which works through hydraulic pressure, allowing the sea urchin to pump water into and out of the tube feet, enabling it to move.

The mouth lies in the centre of the oral surface in regular urchins, or towards one end in irregular urchins. It is surrounded by lips of softer tissue, with numerous small, bony pieces embedded in it. This area, called the peristome, also includes five pairs of modified tube feet and, in many species, five pairs of gills. On the upper surface, opposite the mouth, is the periproct, which surrounds the anus.

 Typical sea urchins have spines about 1 to 3 cm (0.39 to 1.18 in) in length, 1 to 2 mm (0.039 to 0.079 in) thick, and more or less sharp. Sea urchins are dioecious, having separate male and female sexes, although distinguishing the two is not easy, except for their locations on the sea bottom. Males generally choose elevated and exposed locations, so their milt can be broadcast by sea currents. Females generally choose low-lying locations in sea bottom crevices, presumably so the tiny larvae can have better protection from predators. Indeed, very small sea urchins are found hiding beneath rocks.