26o ZOOLOGY FOR MEDICAL STUDENTS chap. 



its shape has become lop-sided, and the appendages of the abdomen are 

 reduced in importance except the sixth of the left side which forms a 

 kind of hook for holding on to the columella of the shell. In the 

 ordinary crabs the abdomen is carried permanently tucked forwards 

 underneath the cephalothorax and in correlation with this and its loss 

 of 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 

 from above downwards, and the Amphipoda, where the body appears to 

 be compressed from side to side so that the animal lies on its side when 

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

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



Fig. 108. 



Zoaea of a Crab iCorystes). X 24. (After Gurney, from The Cambridge Natural History.) 

 Ab, Third 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 Wood-lice are of interest in that they have developed on their flat 

 abdominal appendages fine tubular ingrowths of cuticle which, like the 

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

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

 these the adult female loses all resemblance to a crustacean, being little 

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

 recognizable. 



The Entomostraca 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, 

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

 Simocephalus , which from the small size and transparency of their tissues 

 are excellently suited for demonstrating the essential facts of crustacean 

 structure under the microscope. 



