CRUSTACEANS 



415 



ABDOMeN 



HORAX M^AD 



Whale- Louse {Cyamus ceti) is exceptional among the members of 

 the sub-order in having its body flattened from above downwards, 

 and it is found parasitic on the skin of whales, to which it is 

 enabled to hold fast by 

 means of strongly -hooked 

 legs. The abdomen is re- 

 duced to a mere stump en- 

 tirely devoid of appendages. 

 The weird-looking Skeleton 

 Shrimps {Caprella) are also 

 distinguished by the presence 

 of a much-reduced abdomen. 

 They are found climbing like 

 monkeys among the branch- 

 ing colonies of various zoo- 

 phytes. 



2. Isopods (fig. 256) differ 

 from Amphipods in being 

 flattened from above down- 

 wards, while the abdomen is 

 shortened and bears plate- 

 like appendages. Most of the species are marine, and of these 

 Cirolana, Idotea, and Sphceroma may be taken as typical British 

 genera. Some, however, are 

 fresh -water, as, e.g., the 

 common native Fresh-water 

 Shrimp {Asellus aquaticus), 

 distinguished by its long 

 limbs; and still others are 

 terrestrial, of which the most 

 familiar is the Wood- Louse 

 {Oniscus murarius), com- 

 monly found under damp 

 stones and in similar places. 

 Some of the Isopods are 

 curiously modified to fit them for a parasitic life, and these will 

 be mentioned elsewhere. 



ABDOMEN 



Fig. 255. — Amphipods (enlarged) 



A, Sand-Hopper [Talitnts lociista); 2-8, last seven segments of 

 thorax, the first is fused with the head, B, Whale-Louse {Cyamus 

 ceil), c. Skeleton Shrimp {Ca^reUa.). 



bead. 



Fig. 256. — Isopods (enlarged) 



ll-viii, Free segments of thorax, the first is fused with the 

 A, Cirolana borealis; B, Wood-Louse [Oniscus Tnurarius). 



