304 



LARV.E OF THE TRIPLO ELASTIC A. 



There is usually present at first an uniform covering of cilia ; but 

 in the later larval stages there are almost always formed definite 

 bands or rings of long cilia, by which locomotion is effected. These 

 bands are often produced into arm-like processes. 



The alimentary canal has, typically, the form of a bent tube with 

 a ventral concavity, constituted (when an anus is present) of three 

 sections, viz. an oesophagus, a stomach, and a rectum. The oeso- 

 phagus and sometimes the rectum are epiblastic in origin, while the 

 stomach always and the rectum usually are derived from the hypo- 

 blast 1 . 



To the above characters may be added a glass-like transparency ; 



and the presence of a wid- 

 ish space possibly filled 

 with gelatinous tissue, and 

 often traversed by contrac- 

 tile cells, between the ali- 

 mentary tract and the body 

 Avail. 



Considering the very 

 profound differences which 

 exist between many of 

 these larva?, it may seem 

 that the characters just 

 enumerated are hardly suf- 

 ficient to justify my group- 

 ing them together. It is, 



however, to be borne in 

 FIG. 218. DIAGRAM OF AN EMBRYO OF PLEURO- , T1 j nf i t ], at mv pronnrls for 

 BRANCHIDIUM. (From Lankester.) til at my glOUnclS IO1 



/.foot; of.otocyst; m. mouth; r. velum; ng. d m S SO de P end c l ulte as 

 nerve ganglion; ry. residual yolk spheres; shs. much upon the fact that 

 shell-gland; /. intestine. t l iev constitute a series 



FIG. 219. LARV.E OF CEPHALOPHOROUS MOLLUSCA IN THE VELIGER STAGE. 



(From Gegenbaur.) 



A. and B. Earlier and later stage of Gasteropod. C. Pteropod (Cymbulia). 

 v. velum; c. shell; p. foot; op. operculum; t. tentacle. 



1 There is some uncertainty as to the development of the oesophagus in the Echino- 

 dermata, but recent researches appear to indicate that it is developed from the hypoblast. 



