100 TSOPODA. 
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 Idoteidw, Oniscide, and Anceide are 
destitute of such an appendage, it occurs in the Asellita, 
Spheromide, and Cymotheidz (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 Caprellz), 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- 
