570 



ZOOLOGY 



SECT. 



Isopoda the thoracic segments are produced laterally into large 

 and prominent pleura. 



The eyes (E) are compound and usually sessile : they are, how- 

 ever, stalked in some of the less specialised members of the order, 

 a circumstance which lends support to the view that the sessile 

 eyes have, in this particular group, arisen by the atrophy of eye- 

 stalks. The antennae (ant. 2) as well as the antennules (ant.l) are 

 uniramous, or the former bear a minute exopodite. The first pair 

 of thoracic appendages (mxp) are modified to form maxillipedes, 



FIG. 469. Asellus aquations. A, dorsal; B, ventral view, abd, abdomen; ant.l, antennule ; 

 ant.2, antenna ; bp. brood-pouch ; c.th, cephalothorax : E, eye ; l.l 1.7, legs ; pl.l pl.6, 

 pleopods ; th.2 th.8, free thoracic segments. (After Gerstaecker.) 



which are sometimes united together in the middle line so as to 

 form a sort of lower lip. The remaining seven thoracic appendages 

 take the form of legs (1.1-1.7) which are usually arranged in two 

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

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

 chelse (p. 578) : vestigial expedites are present in some Tanaidacea. 

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

 (Fig. 468, 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. 



