216 NATURAL HISTORY OF BHOMPSTON POND. 



slight spine (d). In other respects, such as the two diverticula 

 of the alimentary canal and the five pairs of thoracic feet, &c., 

 there is a marked resemblance to that genus. 



Chydorus sphaericus (fig. 50) occurs sparingly in places, 

 chiefly on the eastern side of the pond. The body, bearing 

 two very short antennae (a) and a compound eye, is protected 

 by a smooth, almost spherical carapace, produced anteriorly 

 into a long, pointed, and curved beak (b). There is no 

 indication of a groove between the head and the thorax, the 

 latter bearing five pairs of limbs. The alimentary canal is 

 clearly looped (c), but does not give rise to diverticula. The 

 brood-pouch of the female usually contains one, or at the 

 most two, summer eggs (d). The mode of swimming is less 

 jerky than that of the Daphniidae, and its habitat seems more 

 like that of the Ostracoda. 



Yet another species of these little Entomostraca [Alona 

 quadrangularis] (fig. 51) is to be found by carefully examining 

 the debris of the pond. The body is more oval than that of 

 Chydorus, but resembles it in the number of appendages, its 

 alimentary canal (a), and the number of its parthenogenetic 

 eggs or young (b). 



The order Ostracoda is represented by numerous minute 

 carnivorous forms to be found in the sediment. The 

 genus Cypris (fig. 52) is found in plenty, and is characteristic 

 of the group. The unsegmented body is completely 

 enclosed in a bi-valve shell, the two halves of which 

 can be either opened by an elastic ligament, or closed 

 by an adductor muscle, which passes across between the 

 valves. The members of this order have few appendages ; 

 besides antennules (a), antennae (b), mandibles, and two 

 pairs of maxillae, there are only two pairs of stout cylindrical 

 legs, the abdomen being vestigial. The animal moves by 

 lashing the water with its most gracefully pencilled anten- 

 nules, assisted by movements of the antennae and first pair 

 of legs (c), the second pair of legs being employed to sustain 

 the generative organs, which, however, partly project between, 

 and are supported by, the lamellae of the shell. It is note- 



