238 THE INFANCY OF ANIMALS 



water, hither and thither, with wonderful rapidity. Many 

 of these specks, under a microscope, will be found to be 

 Crustacea. Some are adults, belonging to that group of 

 minute species known as Copepoda, which pass the whole 

 of their life swimming at, or near, the surface of the sea ; 

 others will be the larvae of larger species which in their 

 adult stages crawl about on the sea floor. 



Take the case of the larval history of the Common Shore 

 Crab {Carcinus maenas). The youngest stage is that 

 known as the "Zoea," wherein the head and fore-part 

 of the body are covered by a shield, or carapace, with a 

 long spine thrust up from the middle of the back, and 

 another, like a beak, in front. Behind is the long abdomen, 

 which can be freely bent backwards and forwards, and 

 ends in a forked " fin." The eyes are large, and set close 

 to the head, not on stalks, as in the adult. Behind the 

 " beak " are short antennules and antennse, and behind 

 these two pairs of swimming feet, which really answer 

 to a part of the jaw armature in the adult. True feet 

 have not yet made their appearance. 



This zoea stage, as a matter of fact, is preceded by a 

 yet earlier stage ; for when first hatched this youngster 

 lacked the spines on the carapace and some other details. 

 But the zoea stage is assumed a few hours after hatching, 

 by a " moult." It now swims rapidly about in the sea, 

 undergoing several moults. Soon, however, it assumes 

 a new form, such as that shown in the accompanying 

 figure. This is the " Megalopa " stage. Herein the spines 

 of the carapace have vanished, and the legs have appeared 

 — crawling legs under the carapace, and " swimmerets " 

 under the abdomen. Note that this abdomen is stretched 

 out behind the carapace, lobster-fashion. 



Next follows the stage wherein the general shape of the 



