XI 



PHYLUM ARTHROPODA 



585 



two groups, four of them directed forwards and three backwards, or 

 vice versa. The legs end either in simple claws or in large sub- 

 chelae : vestigial exopodites are present in some Tanaidacea. 

 In the female, certain of the legs bear flat plates, the oostegites 

 (Fig. 461, os), probably modified epipodites, which enclose a brood- 

 pouch for the reception of the eggs. In Amphipods the gills are 

 also borne on the legs. 



The abdominal appendages are very different in the two orders. 

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



B 



FIG. 4(52. Asellus aquaticus. A, dorsal ; B, ventral view. aM, abdomen ; ant.l, antennule ; 

 ftiit.2, antenna; bp. brood-pouch; c.th, cephalothorax : E, eye; LI 1.7, legs; j>l.l pl.7, 

 pleopods ; tk.3 th.S, free thoracic segments. (After Gerstaecker.) 



461, 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. 462, pl.3), the former thin and 

 vascular and acting as gills: the sixth pair (pi. 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. 463, 1) is a marine form of glassy 

 transparency, the female of which inhabits a transparent barrel- 

 like structure the test of a pelagic Tunicate in which she 



