﻿130 Aiinala of the South African Museum. 



times quite definitely missing, there are other cases when the ring 

 is incomplete or when coalescence with the preceding joint can only 

 he inferred from a line of suture. The endopod or inner ramus of 

 the uropods may he a single piece or it may be divided into tvvo 

 or three joints of varying relative lengths. The resulting diffeiences 

 are rather easy to observe, and have been, in fact, of much service 

 in classification. But even here perplexities occasionally arise. 

 Among the species of Sympoda earliest described are Bodotrla 

 scorpioidcs (Montagu) and Bodotrla arenosus, Goodsir. These are 

 so much alike that their generic separation is hai'dly to be thought 

 of. Nevertheless the uropod of the former has a two-jointed 

 endopod, while that of the latter is provokingly undivided. 



When the question arises of arranging the families in a natural 

 order, one would probably think precedence appropriate to those 

 which retain the most primitive characters. Among these would be 

 the most complete segmentation of the body and the fullest equip- 

 ment of the segments with their several pairs of appendages. On 

 the first account the families with a distinct telson should stand 

 before those without one. But when the second point is also con- 

 sidered, we find the full complement of five pairs of pleopods 

 combined with entire want of a distinct telson, or in one case with 

 a telson of the smallest type. All other families with the telson 

 distinct have a diminished number of pleopods, varying from three 

 pairs to none. These differences refer only to the male sex, because, 

 so far as at present known, all the females with singular unanimity 

 dispense with pleopods altogether. In some families, however, the 

 males are in this respect like the females. 



The provision of exopods or swimming branches on the peraeopods 

 in the two sexes has its uses for systematic arrangement. But while 

 in the majority of families the adult males have these branches well 

 developed on the first four pair of peneopods, the females are never 

 so well provided, having at most exopods well developed on the first 

 three pairs and a rudiment on the fourth. In both sexes the 

 exopods may be limited to the first pair of peraeopods. For full 

 advantage to be taken of these much-varying characteristics it is 

 obviously important that both sexes should be observed. But, owing 

 probably to the respective habits of these, it not unfrequently happens 

 that new species have to be, or at any rate are, founded on specimens 

 of a single sex, so that the characters of the other sex have to be 

 guessed at or left out of count. 



These are a few of the difiiculties which confront the systeniatist 

 in points the most readily available for his purpose. There are 



