PHYLUM ARTHROPO 



545 



The abdominal appendages are very different in~the^Fwo orders. 

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

 431, sf), the last three peculiar stiff processes used for jumping 

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

 like endo- and exopodites (Fig. 432, pl.3), the former thin and 



B 



;. 432. Asellus aquaticus. A, dorsal; B, ventral view; ant.l, antennule ; ant.2, an- 

 tenna; bp, brood-pouch; c.th, cephalothorax ; E, eye; [l.l 1.7, legs; pl.lpl.7 y pleopods; 

 th.2th.8 ; free thoracic segments. (After Gerstaeckcr.) 



vascular and acting as gills : the sixth pair (pl.G) 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. 433, 1) is a marine form of glassy trans- 

 parency, 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 



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