ISOPODA. 109 



their relations to the other Eclriophthalma, 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 Idoteid^e and Onis- 

 cidse 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 Oniscidge differ from all the rest in having the 

 branchiae transformed into lungs. The Oniscidae, 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 Sphaeromidse 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 

 Idoteidaa 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 



