CHAPTER XII 
ECHINODERMS 
136. General characters—The division of the echino- 
derms includes the starfishes, sea-urchins, serpent- or brittle- 
stars, sea-cucumbers, and crinoids or sea-lilies. All are ma- 
rine forms, and constitute a conspicuous portion of the 
animals along almost any coast the world over. From 
these shallow-water situations they extend to the greatest 
depths of the ocean, and the bodily form possesses a great 
number of variations, adapting them to lives under such 
diverse conditions; and yet there is perhaps no group of 
organisms so clearly defined or exhibiting so close a resem- 
blance throughout. At one time it was thought that their 
radial symmetry was an indication of a close relationship 
to the coelenterates, but more careful study has shown them 
to be much more highly developed than this latter group, 
and widely separated from it. A skeleton is almost always 
present, consisting of a number of calcareous plates embed- 
ded in the body-wall, and often supporting numbers of pro- 
tective spines, which fact has given to the group the name 
Kchinoderm, meaning hedgehog skin. 
137, External features.—The body of a starfish (Fig. 87) 
consists of a more or less clearly defined disk, from which 
the arms, usually five in number, radiate like the spokes 
ofa wheel. At the center of the under side the mouth is 
located, and from it a deep groove, filled with a mass of 
tubular feet, extends to the tip of each arm. Innumerable 
calcareous plates firmly embedded in the body-wall serve 
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