220 ZOOLOGY 



by some of the holothurians. Occasionally, at least, an arm 

 and a small portion of disc seems to have the power of reproduc- 

 ing a new disc and other arms. This power of throwing off arms 

 and replacing them is doubtless a means of protection. 



255. Ecology. — The echinoderms are marine. The larvae 

 are free-swimming, but after the assumption of the adult form 

 they usually become much less active. The crinoids are typic- 

 ally stalked and often attached. The asteroids and echinoids 

 inhabit the bottom of the ocean where they creep more or less 

 slowly. They may be found at almost any depth, from the 

 shallow pools at low tide to the deeper 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. Echinoderms, being slow movers, 

 are compelled to subsist upon such food as the currents or the 

 chance movements of other animals may bring, or upon the 

 debris which falls to the bottom of the sea, or upon such organ- 

 isms as are attached and cannot escape. The starfishes, for 

 example, are a constant menace to the oyster beds. The fact 

 that some starfish are in a measure gregarious makes this all 

 the more serious. It is difficult to see how the starfish 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 importance. The Chinese esteem 

 some species of Holothuria (the trepang, for example) as food. 

 The group appeared early in geological time (Cambrian) and 

 has had very characteristic representatives in all succeeding 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. 



256. 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 com- 



