ECHINODERMATA. 213 



they creep more or less slowly. They may be found at almost 

 any depth, from the shallow pools at low tide to the deepest 

 bottoms. Many of them burrow in the mud and sand, and 

 others (some sea-urchins) have the power of scouring out 

 burrows in the rocks by the action of their spines. Echino- 

 derms, being slow movers, are compelled to subsist upon such 

 food as the currents or the chance movements of other ani- 

 mals may bring, or upon the debris which falls to the bottom 

 of the sea, or upon such organisms as are attached and cannot 

 escape. The star-fishes for example are a constant menace 

 to the oyster beds. The fact that some star-fish are in a meas- 

 ure gregarious makes this all the more true. It is difficult to 

 see how the star-fish can get the oyster from the protection of 

 its shell ; but it manages to get the shell open and clasping its 

 arms about its prey it turns the cardiac portion of its stomach 

 inside-out over the soft part of the oyster and thus leisurely 

 digests it outside its body, so to speak, leaving the empty shell 

 behind. Except for this the group .is of little economic im- 

 portance. The Chinese esteem some species of Holothuria 

 (the trepang, for example) as food. The group appeared 

 early in geological time and has had very characteristic rep- 

 resentatives in all ages up to the present. The changes which 

 have taken place in the echinoderms from one geological age 

 to another are among the most interesting and instructive 

 furnished by the invertebrates. 



250. Classification. Class I., Blastoidea; Class II., Cystoidea. 



(These are both extinct, fossil classes. They comprise stalked and 

 attached forms, and perhaps represent the nearest approach of our known 

 species to the primitive echinoderms.) 



Class III. Crinoidea (feather-stars and sea-lilies}. These forms 

 are less common than in earlier geological times, when they must have 

 been very abundant and very beautiful. They contribute much to the 

 formation of the fimestone of the Palaeozoic. They are usually provided 

 with jointed stalks, by which they may be attached to the bottom. At the 

 summit of the stalk is a central disc with five arms often much branched 

 and bearing lateral pinnules. The anus is on the oral or upper surface, 

 the stalk arising from aboral surface. They are inhabitants of the deep 

 sea. 



Class IV. Asteroidea (star-fishes) (Fig. 95). The asteroids, of which 



