556 



BIPINNARIA. 



M tiller differs from him on this point) to remain open through- 

 out. The further changes in the metamorphosis are not con- 

 siderable. The ciliated bands disappear, and a calcareous ring 

 of ten pieces, five ambulacral and five interambulacral, is formed 

 round the oesophagus. A provisional calcareous skeleton is also 

 developed. 



All the embryonic systems of organs pass in this case 

 directly into those of the adult. 



The metamorphosis of most Holothuroidea is similar to that just 

 described. In Cucumaria (Selenka) there is however no Auricularia stage, 

 and the uniformly ciliated stage is succeeded by one with five transverse 



FIG. 257. THREE STAGES IN THE DEVELOPMENT OF SYNAPTA. A and B 

 are viewed from the ventral surface, and C from the side. (After Metschnikoff.) 



m. mouth; oe. oesophagus; pv, walls of the perivisceral cavity; wv. longitudinal 

 vessel of the water- vascular system ; p. dorsal pore of water-vascular system ; 

 cr. ciliated ring formed round the mouth from parts of the primitive ciliated 

 band. 



bands of cilia, and a prae-oral and an anal ciliated cap. The mouth is at 

 first situated ventrally behind the prae-oral cap of cilia, but the prae-oral 

 cap becomes gradually absorbed, and the mouth assumes a terminal 

 position. 



In Psolinus (Kowalevsky) there is no embryonic ciliated stage, and the 

 adult condition is attained without even a metamorphosis. There appear to 



