THE CRUSTACEA 



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



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



where the maxillipeds are represented by an 



Fig. 13S. unpaired plate carrying a pair of moA'able, un- 



Reducert maxiiii- jointed appendages, representing in all probal)ility 



(After G. o.^Sars.') '"' 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 Platycyamns they are represented only by an unpaired 



plate without any trace of articulations. In Cymnus nodosiis 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 unjointed 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, ^7;) 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 {hr) 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 l^asipodite 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 nieropodite 

 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) 



