Echinoidea
Sea urchins are small, spiny animals that make up the class Echinoidea of the Echinoderm phylum. There are about 950 species of Echinoideas that inhabit all oceans from the intertidal depth to 5000 m deep.The shell of sea urchins is round and spiny, usually from 3 to 10 cm across. The common colors for Echinoideas 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, and eels hunt and feed on sea urchins.
|
Skeletal Structure
In echinoideas, the skeleton is almost always made up of tightly interlocking plates that form a rigid structure, or test, compared with the more flexible skeletal arrangements of starfish, brittle stars, and sea cucumbers that are also members of the same Phylum. Test shapes range from nearly circular, as in some sea urchins, to be flattened, like sand dollars. Living echinoideas are covered with spines, which are movable and anchored in sockets in the test. These spines may be long and noticeable, as in typical sea urchins. In sand dollars and heart urchins, however, the spines are very short and form a soft covering. The mouth of most echinoids is provided with five hard teeth arranged in a circle, which allows them to feed off algae.
|