APPENDAGES OF TRUNK AND TAIL 45 



perseon, has proposed to substitute for it ' btenosome,' 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 chelae is not con- 

 fined to the first pair of so-called peraeopods, 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 perseopods there are developed in great 

 variety of form the branchiaa 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 vulvse, or uterine openings of the 

 female, belong to the sternal, that is the ventral, side of 

 the twelith 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, 



