PHYLUM ARTHROPODA 



571 



The abdominal appendages are very different in the two orders. 

 In Amphipoda the first three are biramous swimming-feet (Fig. 

 468, s.f.), the last three peculiar stiff processes used for jumping 

 (j.f). In Isopods more or fewer of the pleopods have broad plate- 

 like endo- and exopodites (Fig. 469, pl.3), the former thin and 

 vascular and acting as gills : the sixth pair (pi. 6) are either leg- 

 like or aid in the formation of a tail-fin. 



Interesting modifications occur in both sub-orders. Among the 

 Amphipoda, Phronima (Fig. 470, 1) is a marine form of glassy 



3. C a JDP e Ma 



2. Cy a mu s 

 FIG. 470. Amphipoda. 3, a, male ; b, female. (After Gerstaecker, and Bate and Westwood.) 



transparency, the female of which inhabits a transparent barrel- 

 like structure the test of a pelagic Tunicate in which she brings 

 up her young. Caprella (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. 



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

 common Wood-louse (Fig. 471, 1), which is almost unique among 

 Crustacea for its perfect adaptation to terrestrial life. The allied 



