316 



COMPARATIVE ANATOMY 



CHAP. 



each side a basin-like brood capsule serving for the reception of the eggs, formed out 

 of the shaft and its respiratory plate. Each of the limb-bearing trunk segments lying 

 behind the llth carries several (up to 6) pairs of swimming feet, gradually diminishing 



in size, an arrangement which is 

 not yet sufficiently explained. The 

 Branchiopoda (BrancMpus) gener- 

 ally possess 11 pairs of swimming 

 feet. 



The Cladocera (Fig. 215, A), in 

 contradistinction to the Branchio- 

 poda, are distinguished by the 

 small number (4 to 6) of their trunk 

 limbs. The special form of these 

 trunk limbs in some genera recalls 

 the swimming feet of the Branchio- 

 poda, and they carry branchial 

 appendages, especially in Sida and 

 Daphnia. The most anterior trunk 

 feet may, however, become slender 

 and more leg - like, and finally 

 through degeneration of the 

 branchial appendage and the re- 

 spiratory plate may become long 

 seizing feet (Polyphemus, Lepto- 

 dora). 



Ostracoda.- The reduction in 

 the number of limbs here goes 

 even further than in the Cladocera. 



and without branchial appendages 

 The anterior pair 



PIG. 215. Trunk feet of some Entomostraca. A, 

 Daphnia similis O , 2d limb (after Glaus). H, Limno- 

 cythere incisa, last (3d) limb, i.e. 2d trunk limb (after 

 Dahl). c, Apus longicaudatus g, 1st limb (after We find only 2 pairs, which (Fig. 

 Packard). D, Notodelphydae, Doropygus porcicauda 215 ) are j and many .j mted 

 <j> , swimming foot of the 4th pair (after Brady). E, 

 Balanus perforatus, 2d cirrus (after Darwin). 1, 2, 3, 

 4, 5, Endites ; ex, respiratory plate or exopodite ; Ir, gill ; ancl ex P dl tes. 

 / and II, joints of the protopodite. serve as creeping or clinging feet. 



the posterior as cleaning feet. In 



G'ypridina the latter are inserted dorsally on the trunk, and are here long many- 

 jointed appendages (cf. Fig. 193, p. 289). Locomotion is effected chiefly by the limbs 

 of the head. 



Copcpoda (Figs. 194 and 195, pp. 290 and 291 ; Fig. 215, D}. The 4 or 5 pairs of 

 feet are limited to the anterior part of the trunk, which as thorax is opposed to 

 the limbless abdomen. The most anterior pair is inserted on the 1st thoracic 

 segment, which is fused with the head, and is generally unlike the other pairs in its 

 form. The thoracic limbs, as rowing feet, cause the swimming movement of the 

 Copcpoda. They, unlike those of the Phyllopoda, exhibit in a fine typical manner 

 the biramose character, as they consist of a protopodite of 2 joints, an exo- and 

 an endo-podite. The exo- and endo-podites generally have 3 joints (in the Argulidoc, 

 they are long and many jointed) and function as flat oars. Adaptation to a 

 parasitic mode of life in the Copcpoda leads to the reduction and occasionally to 

 the disappearance of the thoracic feet, e.g. in the Chondracanthina the 3d, 4th, and 

 5th pairs of thoracic feet are wanting, and in the Lernceqpodidce all the thoracic 

 feet have disappeared. 



Cirripedia (Fig. 215, E). The trunk extremities of these Crustaceans are biramose ; 

 their exo- and endo-podites are long and many -jointed, and are described as tendril- 



