DEVELOPMENT 



317 



(c, D, e) is formed, the cells becoming differentiated into 

 ectoderm and endoderm, and the ectoderm cells acquiring 

 cilia. The gastrula gradually takes on the form of a peculiar 

 free-swimming larva having a certain general resemblance 

 to the trochos'phere and called a bipinnaria (Fig. 80) : it 

 differs from the adult starfish in showing no trace of radial 

 symmetry, the body being produced into several ciliated 



Fig. 80.— Three stages in the development of the Bipinnaria larva of 

 a Starfish. An, anus ; aor, pre-oral ciliated ring ; 1110, mouth ; por, 

 post-oral ciliated ring. (From Parker and Haswell, after Leuckart 

 and Nitsche. ) 



processes or arms, all bilaterally arranged, and the enteric 

 canal having the form of a curved cylindrical tube, consist- 

 ing of gullet, stomach, and intestine lying in the median 

 plane. The bipinnaria lives a free life for a time, swimming 

 by means of its cilia, and finally, by a complex series of 

 changes, undergoes gradual metamorphosis into the adult 

 starfish. 



