102 ISOPODA. 
less soldered together. There is considerable difference 
in the number of segments composing the tail (pleon), 
the terminal piece of which is often of large size; and 
there is also considerable diversity in the structure and 
functions of the appendages of the tail segments (pleon), 
which never form multiarticulated appendages, as we 
have seen to be the case with the pleopoda, or tail-feet 
of the Amphipoda. 
Except in the aberrant families, the head is generally 
small, and is never confluent with the following segment. 
The eyes are fixed at the sides of the upper surface. 
These organs vary considerably in size in the different 
groups, occupying nearly the whole upper surface of the 
head in the Rocinelw, and being almost obsolete in some 
species in which the tegument of the head, over the place 
of these organs, appears smooth and transparent, whilst 
in other cases the very distinct and large size of the 
hexagonal lenses is very marked. 
The antenne occupy the anterior part of the head, 
and consist of two pairs, generally small, or of but mo- 
derate length; sometimes, indeed, the upper pair are 
rudimentary, asin many of the Oniscide. They generally 
consist of three large basal joints and a multiarticulate 
terminal slender filament ; sometimes, however, the latter 
part is very much reduced in the number of its articula- 
tions, which are enlarged, so as to appear as though regu- 
larly continuous with the basal joints. The first pair of 
antenne, except in the aberrant genus Apseudes, is never 
furnished with a secondary appendage at the base of the 
fourth joint, which is so constantly the case in the Am- 
phipoda (being either large or rudimentary in the larva 
or adult stages, so that it may possibly be a universal 
feature of that order), 
The parts of the mouth are generally strongly deve- 
