ECHINODERMATA 2 19 



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, pelagic, but after the assumption of the 

 adult form they usually become much less active. The crinoids 

 are typically 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 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. 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 and has had very 

 characteristic representatives 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. 



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- 



