230 THE CRUSTACEA 



ischiopodite also bears an endite (the " outer plate," / 3 ), and the 

 remaining four segments form the " palp." The palp is not 

 unfrequently abbreviated by the suppression of one or two of its 

 segments, and the coalescence of the proximal 

 region may involve the basipodites partially or 

 completely. Both these modifications are carried 

 j\ to an extreme in the Hyperiidea (Fig. 138), 



where the maxillipeds are represented by an 

 FIG. 138. unpaired plate carrying a pair of movable, un- 



Beduced maxiiii- jointed appendages, representing in all probability 

 ba ' the ischiopodites with their endites, and a median 

 process corresponding to the coalesced inner 

 plates. In the Cyamidae the maxillipeds are sometimes of 

 normal structure, but they may be greatly reduced (Fig. 135, B), 

 and in Platyeyamus they are represented only by an unpaired 

 plate without any trace of articulations. In Cyaiiius nodosus the 

 interesting observation has been made that the young animals 

 taken from the brood-pouch have well-developed maxillipeds with 

 the full number of segments, although in the adults of this species 

 they are reduced to a pair of un jointed appendages attached to the 

 common basal plate. 



Of the remaining seven pairs of thoracic appendages, the first 

 two are commonly, though not invariably, modified for prehension, 

 and are distinguished as gnathopods (Figs. 132, 134, 135, gn) from 

 the remaining five pairs, the peraeopods, which are generally organs 

 of locomotion. Each limb consists of the usual seven segments. 

 The coxopodite is always short, but is usually expanded externally 

 to form a coxal plate (Fig. 132, ex), sometimes of great size; 

 internally it bears the branchial plate (br) and oostegite when 

 these are present. In the Caprellidea and Ingolfiellidea the 

 coxopodite remains small. In some Hyperiidea it is entirely 

 coalesced with the somite. The basipodite is usually more or less 

 elongated ; the ischiopodite, on the other hand, generally short, 

 contrary to the rule among the Isopoda. The terminal claw is 

 usually coalesced with the dactylopodite. 



The lateral compression of the body in most Amphipoda has 

 lead to a separation of the thoracic legs into an anterior group of 

 four (the two gnathopods and the first two paraeopods) and a 

 posterior of three, which are opposed to each other in the direction 

 of the principal articulations. In the case of the anterior group 

 the limb is bent forwards at the articulation between the ischio- 

 podite and meropodite, and backwards at that between meropodite 

 and carpopodite, and the dactylopodite points backwards ; in the 

 posterior group these directions are reversed and the dactylopodite 

 points forwards except in the case of certain Gammaridea, where 

 the direction of the dactylopodite (but not of the other segments) 



