ISOPODA. 101 



clages that are evidently powerful for swimming purposes. 

 They are biramous, and consist, as in Amphipoda, of three 

 pairs ; moreover, the form of the body and the large size 

 and cheliferous condition of the first, or (as in Apseudes) 

 of the first and second pairs of legs (gnathopoda), 

 remind us of the Amphipodous rather than of the Iso- 

 podous type. 



By Linnasus the animals of this order were arranged 

 almost entirely under the single genus Oniscus, and from 

 their flattened oval form they bear a certain general re- 

 semblance to the Brachyurous Decapods, especially to 

 such anomalous genera as Remipes and Hippa ; whilst 

 the Amphipods, from their compressed or cylindrical 

 form, may be considered more especially as the represen- 

 tives of the Macrourous division. This system of analogy 

 is also supported by the movements of the animals in 

 these several groups, the Brachyura and Isopoda being 

 pre-eminently crawlers ; whilst the Macroura and Am- 

 phipoda are powerful swimmers, both of the last-named 

 sets of animals using the tail as the principal organ of 

 progression. The body of the Isopods is depressed, 

 generally broad, and of an oval form ; in some few 

 cases it is narrow (as in the Apseudes, Arcturus, and 

 Anthura) ; in others, Spha^romidae and Armadillida?, it 

 is constructed so as to be capable of being rolled up into 

 a ball, and in a few it is unsymmetrical, as in the females 

 of certain Bopyridse. The texture of the outer integu- 

 ment is of a firmer consistence than in the Amphipoda ; 

 although in some of the parasitic species it is soft and 

 almost fleshy. The head, except in some of the aberrant 

 families, is distinct from the body, as are the several seg- 

 ments from each other. In the Anceidae, which possess 

 only five pairs of legs adapted for w^alking, the three 

 posterior segments of the body of the female are more or 



