94 REVIEW. 



whole of the available yolk, unci have come to rest bj their peripheral 

 ends against the single-layered ectoderm of the haemal side of the 

 embryo. They thus acquire, relatively to the other cells of the 

 body, a perfectly gigantic size, and bear a curious resemblance 

 to the yolk pyramids of the periblastula. 



With regard to later stages, it is a remarkable circumstance 

 how little of the adult Crayfish's ahmentary canal is archenteric 

 in origin. As every one who has dissected the animal knows, 

 the canal consists first of a short gullet and capacious stomach, 

 lined with chitin, then of a short, thin-walled, easily ruptured, 

 small intestine, not more than an eighth of an inch long, and 

 devoid of a chitinous lining ; and lastly of a widish large intes- 

 tine, separated from the preceding division by an annular ridge, 

 raised internally into several papillose ridges having a shght 

 spiral twist, and lined throughout with a delicate layer of chitin. 

 Now, it seems tolerably certain that only this small intestine is 

 derived from the embryonic midgut, the oesophagus and stomach 

 arising from the foregnt, and the large intestine from the hind- 

 gut; so that almost the whole alimentary canal is ectodermal and 

 not endodermal in origin. 



It is curious, too, to notice the great difference in this respect 

 between two animals so closely allied as the Crayfish and Lobster. 

 In the latter the non-chitinised small intestine extends to within 

 an inch and a quarter of the anus, the large intestine, which has 

 all the characters of the corresponding structure^, in Astacus, 

 forming not more than one sixth instead of fifteen sixteenths of 

 the post-gastric portion of the alimentary canal. 



