EMBRYOLOGY OF THE PODOPHTHALMATA. Ill 



latter animal exceeds the former in size. Besides this marked 

 example, there are numbers of others ; the ova of all the Palce- 

 monidce, or prawns, are far larger in proportion to the size of the 

 animal than the ova of any of the Brachyura; and those of the 

 " burrowing shrimp," Axhis stirhyjichiis^ an animal only three or 

 four inches in length, are even larger than those of the spiny 

 lobster, which is usually over a foot in length. 



It would seem, however, that the size of the ova may to some 

 extent be regulated by the same law that regulates the protective 

 power which the parent Crustacean is able to afford to its ova 

 during development. This, I think, depends, if not entirely, at 

 any rate to a great extent, upon the conditions under which the 

 animal exists ; so that a deep-water species of sluggish habits, or a 

 species that passes most of its life either in sand-banks or mud- 

 banks, will have larger ova with a smaller amount of protection 

 than a species living on the shore, subject to the rough treatment 

 of the surf, or one swimming near the surface, and exposed to the 

 disturbing influence of the waves and wind. As examples of this, 

 we find that the protective segments of Corystes cassivelatmiis^ a 

 Crustacean inhabiting loose sand in deep water, are by no means 

 so developed as those of species which exist under a less quiet 

 condition of things, — those of the PortiinidcB^ or swimming crabs, 

 being very broad, and thus capable of affording the necessary 

 protection to the spavv^n carried beneath. Again, we find that 

 when the abdominal segments are broad, the ligatures by which 

 the ova are connected together, and to the base of the swimmeret, 

 are more slight than when those segments are narrower, in an 

 animal existing under equally favourable conditions. 



The protection referred to consists in the Brachyura of broad, 

 pear-shaped somites which, as we have seen, fold beneath the 

 sternum ; when the ova are exuded, they are covered by this 

 shield, and are besides defended by the beautiful fan-like swim- 

 ming-feet, which also circulate the water through the mass of eggs. 

 Among the Anomoura, the hermit-crabs, Faguridce, living as they 

 do in the dead shells of Mollusca, obtain this somewhat remark- 

 able and artificial protection for their young. The Macrura, 

 having the abdominal somites developed into arched processes, 



