APPENDAGES OF TRUNK AND TAIL 45 
perzon, has proposed to substitute for it ‘ benosome,’ a 
word of precisely the same sense. The epithets chelate 
and sub-chelate are of constant occurrence in descriptions 
of Crustacea. A limb is chelate when it has joints that 
will act together like a pair of tongs. Generally this 
character is produced by the hingeing of the seventh joint 
a considerable way down on the side of the sixth. When 
the seventh joint or finger can be folded back upon the 
sixth, although the latter is not produced into any thumb- 
like process to oppose it, the limb is then said to be sub- 
chelate, the claw being in that case partial, though often 
extremely efficient. The possession of chelee is not con- 
fined to the first pair of so-called pereeopods, although it is 
seldom elsewhere that they attain a monstrous develop- 
ment, They may occur on any of the pairs, and on several 
in the same animal. In connection both with the maxilli- 
peds and the peraopods there are developed in great 
variety of form the branchie or gills, also the plates of the 
marsupium, wherein, in some groups, the eggs are retained 
for a time after their discharge from the ovaries; and 
again, in some groups, the exopods are developed as swim- 
ming organs. ‘The vulve, or uterine openings of the 
female, belong to the sternal, that is the ventral, side of 
the tweltth segment, while the genital openings of the 
male occupy a similar position in the fourteenth segment. 
In those Crustacea which have the basal joints of each 
pair of legs brought close together, the openings in ques- 
tion have been transferred from the wall of the trunk to 
the first joint in each of the last pair of legs in the male, 
and of the antepenultimate pair in the female. 
15, 16,17, 18, 19, 20. The remaining segments belong 
to the tail or caudal portion of the animal, which has been 
termed the pleon or swimming-part, a convenient and 
often a very appropriate name, although on the other 
hand there are plenty of crustaceans which do not and 
cannot use the pleon to swim with. ‘The first five of these 
segments frequently have appendages that are really 
natatory and may properly be called pleopods, swimming- 
feet. But some or all may be wanting, or rudimentary, 
