CLADOCERA 367 



The alimentary canal resembles that of Chirocephalus (p. 358), but 

 the coeca are unb ranched. The food on being swallowed passes direct 

 to the middle part of the mesenteron, where it is digested, and then 

 forwards to the anterior region and the coeca, where the digested 

 products are absorbed and the indigestible residue sent backwards to 

 be formed into faecal pellets in the hinder part of the mid gut. The 

 maxillary gland lies in the carapace. 



The gonads are simple, elongated sacs lying in the trunk and con- 

 tinuous with their ducts, which open in the male on the telson, in the 

 female dorsally behind the last limb. The eggs are yolky. They are of 

 two kinds, "summer" eggs which have relatively little yolk and de- 



p.ch. 



me.gy. 

 -th.3 



Fig. 246. A diagram of a transverse section through the thorax of Daphnia. 

 After Storch. bri. bristles of the fringes which strain out the food ; bri.' bristles 

 of the second pair of thoracic limbs which guard the opening of the median 

 gully; car. carapace; d. dorsal surface of the thorax; fd.gr. food groove; 

 me.gy. median gully or filter chamber ; p.ch. chambers between the limbs : the 

 enlargement and contraction of these chambers by the movements of the 

 limbs set up a pumping action by which water is caused to flow through the 

 bristle fringes from the median gully; pr.ep. proepipodites, playing upon the 

 carapace and closing the pumping chambers at their outer sides; 1/1.2-4, 

 sections through the thoracic limbs, which being directed backwards are cut 

 transversely: each limb underlies that behind it. 



velop rapidly by parthenogenesis in the brood pouch of the mother, 

 and "winter" eggs with much yolk which need fertilization and 

 develop slowly. The winter eggs are fertilized in the brood pouch, but 

 then the cuticle of the carapace, which has thickened, is thrown off 

 as a case — the ephippium — in which they are contained. They go 

 through the early stages of segmentation within a short time, but after 

 this a period of quiescence sets in, during which they may be. dried 

 or frozen without injury. Sexual reproduction takes place at certain 

 times only, normally twice a year. After the winter eggs develop in 

 spring, there are for some half-dozen generations no males, and re- 

 production proceeds by parthenogenesis. Then, about May, a genera- 



