38 



CRUSTACEA — BRANCHIOPODA 



into joints by rings of spines, while in Macrotlirix they are 

 flattened plates. In the males the first antennae are elongated 

 and mobile {cf. Figs. 11, 19). 



The second antennae, the chief organs of locomotion, are 

 biramous in all genera except Holopediuin ; the number of 

 joints in each ramus, and the number of the long plumose hairs 

 with which they are provided, are remarkably constant in whole 

 series of genera, and are therefore useful for purposes of classi- 

 fication. The creatures row themselves by quick strokes of 

 these appendages, the movement being slow and irregular in the 



rounder forms, such as 

 Simocephalus or Daphnia, 

 rapid and well directed in 

 such elongated lacustrine 

 forms as Bythotrephes or 

 Leptodora. 



The mandibles have no 

 palp ; the first maxillae are 

 very small, and the second 

 maxillae are absent (Fig. 9). 

 The carapace varies very 

 much. In most genera (the 

 Calyptomera of Sars) it is 

 Prr. «>• ; / , 7 ^ , ,r ^ large, backwardly - pro- 



JflG. 9. — Simocep/ialus vetulus, female. Ventral . . ° . 



view, without the carapace; A^, A.„ first jecting fold of skin, bent 



i,SrVcrSrr.-lt«"^;„,S downwards at the sides so 

 thoracic appendages. as to form a bivalvc shell, 



enclosing the whole post- 

 cephalic portion of the body, as in Simocephalus (Fig. 10). The 

 eggs are laid into the space between the carapace and the 

 dorsal part of the thorax, both the carapace and the thorax itself 

 being often modified for their protection and nutrition. In a few 

 forms, the Gymnomera of Sars, the carapace serves only as a 

 brood-pouch, which is distended when eggs are laid, but collapses 

 to an inconspicuous appendage at the back of the head when it is 

 empty (e.g. Leptodora, Fig. 24, B?jthotrephes, Fig. 13). In the 

 Calyptomera the surface of the carapace is frequently provided 

 with a series of ridges, which may be parallel, rarely branching, as 

 in Simocephalus ; or in two sets which cross nearly at right angles, 

 as m Daphnia ; or so arranged as to form a hexagonal pattern, as 



