ISOPODA. 101 
dages 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 Linneeus 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, Spheromide 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 Bopyride. 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 Anceidz, which possess 
only five pairs of legs adapted for walking, the three 
posterior segments of the body of the female are more or 
