534 



ZOOLOGY 



SECT. 



wards in Isopods (Fig. 462). The normal malacostracan number 

 of segments is present, but the first thoracic segment is always 

 united with the head, so that the apparent head is really an incom- 

 plete or partial cephalothorax (c.th). In the Tanaidacea (Tanais, 

 &c.) the second segment of the thorax also unites with the head, 

 and such forms — sometimes included under a distinct sub-order, 

 Anisopoda — form a transition to the other Malacostraca, and 

 especially the Cumacea. In the Amphipoda and Isopoda, the pos- 

 terior seven thoracic segments (th.2 — th8) are free, and those of the 

 short abdomen are usually free in Amphipods (Fig. 461, ahd, 



th.Z 



th.S 



Fig. 461. — Gammarus neglectus. abd.l — abd.U, abdominal segments; ant.1, antennule ; 

 ant. 2, antenna ; eth. cephalothorax ; E. eye ; ,/'. /. 1, first jumping foot ; I. 1 — I. 7, .legs ; 

 mtp. maxillipede ; o.i. oostegite ; ov. ova; s.f.l, first swimming foot ; tli.S—th..!, free thoracic 

 segments. (After Gcrstaeckcr.) 



1-6), often more or less fused in Isopods (Fig. 462, 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, or the former bear a minute exopodite. The first pair 

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

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

 form a sort of lower lip. The remaining seven thoracic append- 

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



