Skin of thorns. The spherical or disc-shaped bodies consist of limestone plates interlocked with each other. These limestone plaques have articulated spines.
There are two holes in the upper and lower poles of the sea urchin, which perform the functions of anus and mouth. Eggs or sperm come out of a hole in the five limestone plates on the anus side. The inner structure of the sea urchin moving with bulb legs is quite complicated.
The mouth consists of 5 teeth attached to an organ made of muscles and calcareous bones called the Aristotle lantern. Right next to the mouth are gills. The intestine is in the form of two opposite folds. The nerve center is in the form of a pentagonal ring adjacent the wall membrane around the mouth. The 5 large nerves exiting this ring extend to the anus at the lower pole.
The sea urchin is a separate sexual egg after fertilization in water and undergoes a series of metamorphosis. He lives from shallow shores to deepest places, sometimes in sand, sometimes among hard rocks. The shell of large sea urchins is used to make ornaments.
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