288 ALIMENTARY CANAL. 



gus. The passage of the yolk from the external to the internal yolk-sack is 

 probably largely due to the contractions of the former. 



The external yolk-sack is not vascular, and probably the absorption of 

 the yolk for the nutrition of the embryo can only take place in the internal 

 yolk-sack. The most remarkable feature of the Cephalopod yolk-sack is the 

 fact that it lies on the opposite side of the alimentary tract to the yolk cells, 

 which form a rudimentary yolk-sack in such Gasteropoda as Nassa and 

 Fusus. In these forms, the yolk-sack is at first dorsal, but subsequently is 

 carried by the growth of the liver to the right side. In Cephalopoda on the 

 contrary, the yolk-sack is placed on the ventral side of the body. 



What is known of the development of the alimentary tract in 

 the Polyplacophora has already been mentioned. 



In the Lamellibranchiata (Lankester, No. 239), the mesen- 

 teron early grows out into two lateral lobes which form the liver, 

 while the part between them forms the stomach. 



In Pisidium the intestine is formed from the original pedicle 

 of invagination, which remains permanently attached to the 

 epiblast. The stomodaeum is formed by the usual epiblastic 

 invagination, and becomes the mouth and oesophagus. The 

 development of the crystalline rod and its sack do not appear to 

 be known. In the adult the sack of the crystalline rod opens 

 into a part of the alimentary tract which appears to belong to 

 the mesenteron. Were however the development to shew them 

 to be really derived from the stomodaeum they might be inter- 

 preted as rudiments of the organ which constitutes the odonto- 

 phore and its sack in cephalophorous Mollusca an interpretation 

 which would be of considerable phylogenetic interest. 



BIBLIOGRAPHY. 



General. 



(238) T. H. Huxley. "On the Morphol. of the Cephal. Mollusca." Phil. 

 Trans. 1853. 



(239) E. R. Lankester. " On the developmental history of the Mollusca." 

 Phil. Trans. 1875. 



(240) H. G. Bronn and W. Keferstein. Die Klassen n. OrJnungen d. Thier- 

 reichs, Vol. III. 1862 1866. 



Gasteropoda and Pteropoda. 



(241) J. Alder and A. Hancock. "Devel. of Nudibr." Ann. and Magaz, 

 Nat. Hist., Vol. Xir. 1843, 



