101 
CHAPTERS cv. 
THE SEA-URCHINS. 
THE asterias resemble stars; the Zchinz or sea-urchins may be 
likened to melons; yet both belong to the same class—the 
Echinoderms. 
The sea-urchins are enclosed in a calcareous kind of shell, 
which is generally globular or egg-shaped, but sometimes flat- 
tened. This shell, or carapace, is really built up of polygonal 
plates, which adhere by their edges to each other. The plates 
are so arranged that the shell is divided into vertical zones—hence 
its resemblance to the melon. These zones are of two kinds, one 
being very much larger than the other; the plates of the larger 
zones are covered with sharp spines, which are movable, and serve 
at once for protection and locomotion. The plates of the smaller 
zones are pierced with pores, from which issue filaments, by which 
the animal breathes and walks. 
In the edible sea-urchin (Spherechinus esculentus) the shell is 
composed of 10,000 distinct pieces, so admirably and firmly united, 
that the whole appears but one piece. The prickly spines are 
often very numerous; they cover and protect the shell. From 
these bristles, the animal has been named the sea-hedgehog. Its 
scientific name is derived from éxivos, which was given to the 
creature by Aristotle, from the evident resemblance the shell of 
the echinus, denuded of its spines, bears to a vase. 
In one species as many as 2,000 bristles have been counted ; 
in the edible sea-urchin there must be at least 3,000. These 
appendages entirely cover and hide the calcareous tunic which 
envelops the animal, like the numberless pearls which covered 
the famous habit of St. Simon—the material was of silk, but it 
could not be seen. The bristles of the sea-urchin present, at 
their base, a small hollow head, which has a compression on its 
lower surface, thus forming a cavity which fits a tubercle on 
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