100 ISOPODA. 



of the legs, is that presented by the structure of the 

 appendages of the tail, which in the Isopods support 

 organs of respiration, varying considerably in their for- 

 mation in the different groups, instead of serving, as in 

 the Amphipods, for purposes chiefly of locomotion. 



Another character which was employed by Latreille to 

 distinguish the majority of these animals from the Am- 

 phipoda, namely, the want of an articulated appendage to 

 the hard horny mandibles, is by no means a constant one, 

 since although the Idoteidae, Oniscidae, and Anceidae are 

 destitute of such an appendage, it occurs in the Asellita, 

 Sphaeromidae, and Cymotheidae (of which last we possess 

 no hitherto recorded representative in these islands). 



With the Amphipoda the Isopoda are closely connected 

 through the aberrant families of each order. 



In the genus Cyamus the lowest form of Amphipod 

 with which we are at present acquainted, , the Isopodal 

 appearance is very considerable ; so great indeed that at 

 one time the genus was grouped with this order ; and, 

 in fact, it can only be separated from it by virtue of the 

 branchial organs that are attached to the body ; and even 

 these are reduced to two pairs only. In Cyamus also (as 

 in the Caprellae), the head is confluent with the first seg- 

 ment of the body, and the tail is reduced to a rudimen- 

 tary condition. 



If we turn to the genus Tanais, among the aberrant 

 Isopods, we have an animal that is capable of being se- 

 parated from the Amphipoda only by virtue of its caudal 

 appendages. The head in this genus is confluent with 

 the first segment of the body ; and a pair of branchial 

 vesicles (organs especially characteristic of the true Am- 

 phipoda) are attached to the third pair of pereiopoda (or 

 fifth pair of feet), and the natatory or tail-feet, instead 

 of being simple branchial plates, are foliaceous appen- 



