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 Rocinelae, 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 antennse 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, as in many of the Oniscidse. 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 

 antennse, 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- 



