544 



ZOOLOGY 



SECT. 



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

 terior seven thoracic segments (th. th.8) are free, and those 

 of the short abdomen are usually free in Amphipods (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.%) as well as the antennules (ant.l) 

 are uniramous. The first pair of thoracic appendages (mscp) 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 (LI -1.7) 



C-.'tft. 



Ih.Z 



th.8 



FIG. 431. Gammarus neglectus. aM.l nbd.6, abdominal segments; ant.l, antennulc 

 ant. 2, antenna; cth. cephalothoi ax ; E. eye ; j. /. 1, first jumping foot; /. ll. 7, legs 

 mxp. maxillipede ; os. oostegite ; ov. ova ; s.f.l, first swimming foot; tk.3th.2,free thoraci 

 segments. (After Gerstaecker.) 



which are usually arranged in two groups, four of them directe 

 forwards and three backwards, or vice versa. The legs end eithei 

 in simple claws or in large sub-chela3 : vestigial exopodites ai 

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

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

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

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



