544 



ZOOLOGY 



SECT. 



the other Malacostraca, and especially the Cumacea. The pos- 

 terior seven thoracic segments (///..? tk.S) are free, and those 

 of the short abdomen are usually free in Amphipgds (Fig. 431, 

 aid. 1-6), often more or less fused in Isopods (Fig. 432, aid). In 

 some 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 antennas (ant. 2} as well as the antennules (ant.l) 

 are uniramous. The first pair of thoracic appendages (mxp) are 

 modified to form maxillipedes, which are sometimes united to- 

 gether 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) 



ih.Z 



th.S 



17 



FIG. 431. Gammarus neglectus. ab<i.l^l<l.C, abdominal segments ; ant.l, antemiulc ; 

 ant. .', antenna; cth. cephalothoiax ; E. eye; /. /. 1, first j limping foot; I. II. 7, legs; 

 ,11.',: maxillipede; os. oostegite ; or. ova ; *.j'.l, fiist swimming foot; th.S th.2, free thoracic 

 segments. (After Gerstaecker.) 



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-chelae : vestigial exopodites are 

 present in some of the Anisopoda. In the female, certain of 

 the legs bear flat plates, the oostegitcs (Fig. 431, os), probably modi- 

 fied epipodites, which enclose a brood-pouch for the reception of 

 the eggs. In Amphipods the gills are also borne on the legs. 



