586 



ZOOLOGY 



SECT. 



brings up her young. Gaprella (3) is a singular creature in which 

 the abdomen is quite vestigial, and the rest of the body, as well as 

 the appendages, extremely slender. It creeps about on colonies 

 of Hydrozoa and Polyzoa, to the branches of which its own form 

 and colour are so closely assimilated as to render it difficult of 

 detection. The allied Cyamus (Whale-louse, 2) is parasitic on the 

 skin of whales : it also has a vestigial abdomen, but the body 

 -exceptionally among Amphipods is broad and depressed, and 

 the legs are curiously swollen. 



3. C a |D r e I I a 



""- J 



2. C y a m u s 

 FIG. 463. Amphipoda. 3, a, male ; I, female. (After Gerstaecker, and Bate and Westwood. 



Among the Isopoda, one of the most interesting forms is the 

 common Wood-louse (Fig. 464, 1\ which is almost unique among 

 Crustacea for its perfect adaptation to terrestrial life. The allied 

 " Pill-bugs " (Armadillidium, f?) have the habit of rolling them- 

 selves up into a ball when disturbed. Cymothoa and its allies are 

 large species (6-8 cm. in length) parasitic in the mouths of Fishes, 

 where thevhold on to the mucous membrane with their short, clawed 



/ 



legs: their mouth-parts are often modified for sucking. In the Bopy- 

 rini, found in the gill-cavities of various Crustacea, parasitism is 

 accompanied by great degeneration and asymmetry (5) as well as 

 by a notable degree of sexual dimorphism, the males (3, b, m] being 

 very small and permanently attached to the bodies of the females. 

 Lastly, in Cryptoniscus, parasitic on Crabs, the adult female (4> b) has 



