304 



LARVJi OF THE TRIPLOBLASTICA. 



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'. 



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 

 wall. 



Considering the very 

 profound differences which 

 exist between many of 

 these larvoe, 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 

 mind that my grounds for 

 doing so depend quite as 

 much upon the fact that 

 they constitute a series 



Fig. 218, Diagbam of an embryo of Pleuro- 

 BBANCHiDiuM. (From Lankester.) 



/.foot; ot, otocyst; m. mouth; r. velum; ng. 

 nerve ganglion ; ry. residual yolk spheres ; shs. 

 shell-gland; i. intestine. 



Fig. 219. Laev^ of Cephalophobous Mollusca in the veligeb 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. 



* There is some uncertainty as to the development of the cesophagus in the Echino- 

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



