PHYLUM ECHINODERMATA 211 



(Fig. 150, A, stom), oesophagus (oes), stomach (st), intestine 

 (int), and anus. From the digestive tract two ccelomic sacs 

 (coe) are budded off; these develop into the body-cavity, 

 water-vascular system, and other ccelomic cavities of the 

 adult. 



The larvae of the different classes have been given names as 

 follows: those of the ASTEROIDEA are called Bipinnaria (Fig. 

 150, B); OPHIUROIDEA, Ophiopluteus (Fig. 150, C); ECHINOIDEA, 



FIG. 151. Larval Echinoderms. A, a larval Echinoid (Echinopluteus). 

 j, frontal area; 2, preoral arm; 3, postoral arm; 4, anterior; 5, posterior trans- 

 verse portion of ciliated band; 6, unpaired posterior arm; 7, anal area; 

 8, postero-lateral arm; g, oral area; 10, postero-dorsal arm; n, antero-dorsal 

 arm; 12, antero-lateral arm. 



B, a larval Holothurioid (Auricularia stelligera). i, frontal area; 2, preoral 

 process; j, anterior; 4, posterior portion of ciliated band; 5, postoral process; 

 6, anal area; 7, postero-lateral process; 8, postero-dorsal process; Q, oral de- 

 pression; 10, dorso-median process; n, antero-dorsal process. (From Sedg- 

 wick, after J. Miiller.) 



Echinopluteus (Fig. 151, A); and HOLOTHURIOIDEA, Auricularia 

 (Fig. 151, B). The adults which develop from these larvae are, 

 as we have seen, radially symmetrical, although many of them, 

 notably the HOLOTHURIOIDEA, are more or less bilateral in struc- 

 ture. The bilateral condition of the larvae indicates that the 

 ancestors of the echinoderms were either bilaterally symmetrical 

 or that the larvae have become adapted to an active life in the 

 water. 



