ISOPODA. 109 
their relations to the other Edriophthalma, is by no 
means satisfactory. If the organs of locomotion and 
those of manducation and respiration, together with the 
relative position of the latter, are regarded as affording 
the primary distinctions between the Amphipoda and 
Tsopoda, we must regard those Isopods as pre-eminently 
typical of the order which possess the greatest uniformity 
in the structure of the legs, and which have the breath- 
ing apparatus affixed to the underside of the tail instead 
of the body, and which are destitute of a palpiform ap- 
pendage to their mandibles. The Idoteide and Onis- 
cide fulfil these conditions most completely, the former 
agreeing with the great majority of the order in having a 
branchial apparatus fitted for the use of aquatic existence, 
whilst the Oniscidz differ from all the rest in having the 
branchiz transformed into lungs. The Oniscidz, more- 
over, are terrestrial, or, at least, found in damp situations, 
where they breathe the free air, and these can scarcely, 
therefore, be regarded as pre-eminently typical of an ex- 
tensive series of groups, all the rest of which are aquatic. 
The great family of Spheromide fulfil the greater por- 
tion of the conditions of a typical group set down above, 
but they have a palpiform appendage to the mandibles ; 
on the whole, however, they appear to constitute a well- 
marked ovate or globose type of the order, whilst the 
Idoteide have a more elongated general form. We 
therefore accept Ligia, holding, as it does, an interme- 
diate position between the aquatic and air-breathing 
Isopoda, as the most typical genus of this order. All 
the remaining tribes are more or less differentiated. 
As the British species of Isopoda contain representa- 
tives of nearly all the known groups into which the order 
has been subdivided, it will be necessary to notice the 
