2 6o ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



its sha] ome lop-sided, and the appendages of the abdomen are 



i i n importance except the sixth of the .left side which forms a 



; I 1( ,,,k t..r holding on to the columella of the shell. In the 



ordinary crabs the abdomen is carried permanently tucked forwards 



un.lerneath the cephalothorax and in correlation with this and its loss 



,,f function in swimming it has become greatly reduced in size in the 



adult although in the Zoaea stage it is of full size (Fig. 108). 



In the ISOPODA, characterized by the form of their body, flattened 



irom above downwards, and the AMPHIPODA, where the body appears to 



.pressed from side to side so that the animal lies on its side when 



taken irom the water, there is no carapace. Among the Isopoda are a 



number of genera which have taken to a terrestrial existence, although 



FIG. 108. 



ib (Corystes). X 24. (After Gurney, from The Cambridge Natural History.) 

 .ird segment of abdomen ; An, First antenna ; E, eye ; M, first maxilliped. 



a damp atmosphere is still essential to their life. Many of these Slaters 



or \Yood-lice are of interest in that they have developed on their flat 



alxlominal appendages fine tubular ingrowths of cuticle which, like the 



ae of insects, serve for breathing air. Other members of the group 



have taken to living parasitically upon other crustaceans and in some of 



he adult female loses all resemblance to a crustacean, being little 



han a bag of eggs, although in its young stages its character is clearly 



ble. 



i IMOSTRACA include an immense variety of crustaceans of 



which only the chief types can be mentioned. Amongst the BRANCHIO- 



PODA are included a number of very common short-bodied creatures, 



ines termed Water- fleas, belonging to such genera as Daphnia and 



'vW//v. which from the small size and transparency of their tissues 



itly suited for demonstrating the essential facts of crustacean 



structure under the microscope. 



