248 



MARINE INVERTEBRATES 



Zoea of Cancer irroratus. Last stage 

 before it changes to the megalops con- 

 dition. 



The muscles are white bundles of fibers, and are in strips, 

 which is an indication of power and activity. There are four 

 important sets : twisted ventral muscles which bend the tail and 

 are particularly large and strong, those which straighten the tail, 



those moving the appendages, and 

 those which work the gastric mill. 



The eggs, after being discharged, 

 are attached to the abdominal legs of 

 the mother by a kind of cement, or 

 they are carried in pouches attached 

 to the thorax. In these positions they 

 mature, and hatch at different stages 

 of development in different species. 

 Some emerge with three pairs of legs, 

 and are known as Nauplius; some are 

 Zoea, having a carapace and abdominal 

 segments, but no abdominal appendages. Another stage is Megalops, 

 with large stalked eyes. Others are hatched as miniature adults. 

 These names were given when the embryo stage was not recog- 

 nized and the larvae were thought to be distinct species. 



The growth of the animal is effected by moulting. The Crus- 

 tacea are named from the crust-like covering 

 which envelops them. It is a horny material, 

 called chitin, in which are deposited particles 

 of carbonate of lime, making a rigid envelop 

 which would prevent all freedom of motion, 

 were it not that there are spaces free from 

 lime, and thus flexible joints are left. When 

 the animal expands it throws off this hard 

 covering and secretes a new and larger one. 

 As the time of moulting approaches, the old 

 covering becomes loosened, and a delicate 

 new one is formed beneath it. The old shell 

 splits open across the back just behind the 

 carapace, and the soft animal withdraws first its cephalothorax 

 and then its abdomen, leaving the cover complete, including even 

 the covering of the eyes and the lining of the stomach. The 



Megalops stage of Cancer 

 irroratus, just after change 

 from zoea stage. 



