40 THE CRUSTACEA 



the flabellum. This view, however, is not accepted by Sars, 

 who regards the distal setose plate as the sixth endite and 

 supposes that the flabellum is wanting in the Anostraca. Sars's 

 interpretation gains some support from a comparison with the 

 Conchostracan type of limb, where the sixth endite has much the 

 same position as the setose plate of the Anostraca. It is further 

 supported by the fact that, in the development of the limbs of 

 Apus, the flabellum only appears after some of the endites have 

 become marked off, while in Branchipus the first differentiation to 

 take place in the limb-buds is a bifurcation defining the distal setose 

 plate from the terminal lobe of the inner edge. In any case it 

 seems certain that the external basal plate or plates of the 

 Anostraca are new formations unrepresented in the Notostraca and 

 Conchostraca, and not, as Lankester supposed, homologous with 

 the branchia (or bract) of these groups. 



In the Cladocera the structure of the limbs is still more difficult 

 to interpret. In the Ctenopoda (Fig. 24, A), where the six pairs 

 of "thoracic" appendages are comparatively uniform, the inner 

 margin forms a small gnathobasic lobe followed by a broad lobe 

 carrying a comb- like series of long setae. A distal lobe may 

 correspond to the flabellum of Apus and Estheria, or perhaps to the 

 sixth endite. The branchia is present and has the usual characters. 

 In the Anomopoda there is considerable differentiation among 

 the members of the series. In Daphnia, for example, the first limb 

 (Fig. 24, B) is obscurely segmented and the five lobes on the inner 

 edge are slightly developed. The second limb (Fig. 24, C) has the 

 gnathobase enlarged and has an outer branch regarded as the 

 exopodite. Both of these limbs are adapted by the possession of 

 long curved setae to aid in the prehension of food. The third 

 (Fig. 24, D) and fourth pairs are characterised by the great 

 development of the proximal endite with its comb -like row of 

 setae. They serve to keep a current of water flowing between 

 the valves of the shell for the purposes of respiration and feeding. 

 The last pair in Daphnia are greatly reduced (Fig. 24, E). 



In the Gymnomera, and especially in Leptodora (Fig. 19), the 

 trunk-limbs have lost the characteristic leaf-like shape and have 

 become cylindrical, elongated, and divided into four well-marked 

 segments, without any trace of endites or exites, serving only for 

 seizing and holding prey. 



Modifications of certain of the trunk-limbs subservient to 

 the processes of reproduction are found in all divisions of the 

 Branchiopoda. In the males of most of the Conchostraca the first 

 two pairs have the terminal portion modified into a cheliform 

 clasper (Fig. 1 7, B), but in Limnetis and in the Cladocera only the 

 first pair is so modified. Sexual modifications of trunk-limbs in the 

 female are found only in the Notostraca and in some Conchostraca. 



