258 



COMPARATIVE ANATOMY 



CHAP. 



of the foot, in the same way, is often 



Fio. 214. Embryo of Helix Waltoni (4 mm. 

 long), from the right side (after P. and F. 

 Sarasin). 1, Cephalic vesicle ; 2, upper (optic) 

 tentacle ; 3, eye ; 4, lower tentacle ; 5, oral 

 lobe ; 6, sensory plate ; 7, podocyst. 



Ill the larva of the gynmosomatou 

 rings are developed on the body. 



widened into a pulsating pedal vesicle or 

 podocyst. Towards the end of larval life 

 the cephalic and pedal vesicles and other 

 similar "larval hearts" degenerate. 



The embryonic shell is either retained 

 throughout life or is thrown off at an early 

 stage, and replaced by the rudiment of the 

 definitive shell. Even a second temporary 

 shell occasionally attains development. 



It must again be noted that shell-less 

 Gastropods, to whatever natural division 

 they belong, pass through a typical Yeliger 

 stage, and at the older Yeliger stage have 

 a distinctly demarcated coiled visceral 

 dome, with a corresponding shell, and 

 usually an operculum on the metapodium. 

 s Pteropoda three postoral accessory ciliated 



C. Scaphopoda. 



Ontogeny of Dentalium. Segmentation, in these animals, leads to the formation 

 of a cceloblastula, from which a coslogastrula arises by imagination. The blast opore 

 at first lies posteriorly on the ventral side, but 

 gradually shifts, as in Chiton, more and more 

 forward along the ventral side. The stomodseum 

 arises as an ingrowth of the ectoderm, the blasto- 

 pore nevertheless remaining open. A typical 

 Molluscan Trocophora is developed, although no 

 primitive kidney has been found. The velum is 

 a thick ridge round the body of the long oviform 

 larva. This ridge consists of three rings of very 

 large ectoderm cells, each row carrying a circle 

 of long cilia. The shell gland spreads out at an 

 early stage, its lateral edge soon growing out 

 ventrally and posteriorly as the mantle fold. 

 The free edges of the two folds fuse at a later 

 stage below the body. The anus forms very late. 

 The development of the cerebral and pedal 

 ganglia and of the auditory organ has been 

 specially carefully observed. On the ventral 



side of the pretrochal area, in front of the velum 



hours old, posterior and lower aspect 



and behind the tuft of cilia, two symmetrical (aftei . K owalevsky). i, Cephalic tuft ; 



invaginations of the ectoderm form the cephalic 2, rudiments of the cerebral ganglion 



sacs or tubes. These become constricted from the (cephalic tubes); 3, velum, consisting 



ectoderm at a later stage, their lumen gradually of three rows of cilia; 4, mouth, hidden 



,..,. . .,.,"-. ,, under the ridge of the velum ; 5, mantle 

 narrows and finally disappears, while their walls 



Fio. 215. Larva of Dentalium, 37 



become thick and multilaminar by the con- 



tinuous growth of the cells. The two cell masses which thus arise become connected 

 in the middle line above and below the cesophagus, and form the cerebral ganglion. 

 The otocysts arise at the base of the pedal rudiment on each side as ectodermal 

 epithelial pits, which soon become detached from the ectoderm in the form of 

 epithelial vesicles, Immediately beneath these auditory vesicles, certain ectoderm 



