2IO 



COLLEGE ZOOLOGY 



(Fig. 148). The branches may be equal, or one large and the 

 other small; in the latter case the smaller branch is called a 

 pinnule. 



Some authors place the class Crinoidea in the subphylum 

 Pelmatozoa along with four classes of fossil echinoderms, the 

 Thecoidea (Fig. 149, A), Carpoidea (Fig. 149, B), Cystoidea 

 (Fig. 149, C), and Blastoidea (Fig. 149, D). 



7. Development of Echinoderms 



In most of the echinoderms, the eggs pass through a ciliated 

 blastula stage, a gastrula stage, and a larval stage, which, in the 



Fig. 150. — Larval Echinoderms. A, a young larval echinoderm. coe, coelom; 

 int, intestine; oes, oesophagus; st, stomach; stom, stomodaeum. B, a larval 

 Asteroid, Bipinnaria elegans. i, frontal area; 2, preoral arm; 3, anterior, 

 4, posterior transverse portion of ciliated band; 5, postoral; 6, postero-lateral; 

 7, postero-dorsal arm; 8, anal area; q, oral depression; 10, antero-dorsal; 

 11, ventro-median, 12, dorso-median arm. C, a larval Ophiuroid (Ophio- 

 pluteus). a, anus; d, antero-lateral arm; d', postero-lateral arm; e, postoral 

 arms; g, postero-dorsal arm; m, mouth. (A, from the Cambridge Natural 

 History; B and C, from Sedgwick, — B, after Mortensen; C, after Muller.) 



course of from two weeks to two months, metamorphoses into an 

 adult. The larva? (Fig. 150, A) of the four principal classes of 

 echinoderms resemble one another, but are nevertheless quite 

 distinct. They are bilaterally symmetrical, and swim about by 

 means of a ciliated band which may be complicated by a number 

 of arm-like processes. The alimentary canal consists of a mouth 



