316 



COMPARATIVE ANATOMY 



CHAP- 



behind the 11th carries several (up to ( 



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 



) pairs of swimming feet, gradually diminishing 

 in size, an arrangement which is 

 not yet sufficiently explained. The 

 Branchiopoda (Branchipus) gener- 

 ally possess 11 pairs of swimming 

 feet. 



The Claclocera (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 

 Daplinia. The most anterior trunk 

 feet may, however, become slender 

 and more leg -like, and iinally 

 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. 

 We find only 2 pairs, which (Fig. 

 216, B) are long and many-jointed 

 and without branchial appendages 

 and exopodites. The anterior pair 

 serve as creeping or clinging feet, 

 the posterior as cleaning feet. In 

 Oypridina the latter are inserted dorsally on the trunk, and are here long many- 

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

 of the head. 



Copepoda (Figs. 194 and 195, pp. 290 and 291 ; Fig. 215, X>).— The 4 or 6 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 

 Copepoda. 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 Argulidoe 

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

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

 the di.sappearaiioe of the thoracic feet, e.g. in the Clioiulracanihina the 3d, 4th, and 

 6th pairs of thoracic feet are wanting, and in the Lernceopodidce all the thoracic 

 feet have disappeared. 



Cirripcdia (Fig. 215, E). — The ti'unk extremities of these Crustaceans are biramose ; 

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



Fia. 215. — Tnink feet of some Entomostraca. A, 

 Daphnia simills O , 2d limb (after Claus). B, Linmo- 

 cytliere Inclsa, last (3d) limb, i.e. 2d trunk limb (after 

 Dahl). C, Apus longloaudatus ±, 1st limb (after 

 Paotard). B, NotodelpliydoB, Doropygus porcioauda 

 o , swimming foot of the 4tli pair (after Brady). E, 

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

 4, 5, Endites ; ex, respiratory plate or exopodite ; &r, gill ; 

 / and U, joints of the protopodite. 



