Tribe 1. 



CHELIFERA. 



Body generally slender, nearly cylindric in form. Cephalon and the 1st 

 segment of mesosome coalesced, forming together a kind of carapace, which on 

 each side contains a small branchial cavity. The 6 other segments of mesosome 

 well defined, with the coxal plates small or inconspicuous. Metasome generally 

 composed of 6 segments, the 5 anterior short, subequal, the last much the largest. 

 Eyes distinct or wanting. Superior antennae generally simple, sometimes, however, 

 provided with a distinct secondary appendage; inferior ones smaller than the 

 superior and issuing immediately beneath them. Mandibles with or without palps. 

 Anterior maxilla? provided with a reflexed, setiferous palp; posterior ones very 

 small, often quite rudimentary. Maxillipeds more or less coalesced at the base, 

 and each having outside a membranous epignath projecting within the branchial 

 cavity. 1st pair of legs very strong, curving anteriorly, and each terminating 

 in a cheliform hand; 2nd pair sometimes unlike the succeeding ones, wdnch are 

 simple, ambulatory. Pleopoda, when present, comparatively small, natatory, rami 

 lamelliform. Uropoda terminal, consisting of a short basal part and one or two 

 terminal filaments. Sexual difference often very pronounced. 



Remarks. — This is certainly a very anomalous group, differing, as it 

 does, in certain particulars very markedly from the typical Isopoda, and exhibiting 

 some points of resemblance to apparently widely distant crustacean orders, for 

 instance the Cumacea. For this reason it has been proposed by some authors 

 to remove this group altogether from the Isopoda, and to regard it as a distinct 

 order. I do not find, however, that such an arrangement affords any real ad- 

 vantage, and as the present forms agree in many other respects with the Isopoda, 

 I prefer to retain them within that order as an anomalous tribe. The most 

 striking external feature is undoubtedly the peculiar modification of the 1st pair 



