THE BRANCHIOPODA 47 



Reproductive System. The gonads are generally paired, but in 

 some Cladocera they may be united in the middle line. In the 

 Notostraca they are much ramified, but in the other Branchiopoda 

 they are of simple tubular form. Probably all Branchiopoda are 

 of separate sexes. In the Notostraca Bernard has described testi- 

 cular tissue in the ovary, but the occurrence of normal functional 

 hermaphroditism is still unproved. 



In the Notostraca the genital ducts open on the llth trunk- 

 somite, and this is probably also the case in all the Conchostraca. 

 In the majority of the Anostraca the first two apodous somites, 

 namely, the 12th and 13th of the trunk, are more or less com- 

 pletely fused, forming a " genital segment " on which the genital 

 ducts open. The development indicates that the openings prob- 

 ably belong to the 12th somite. In Polyartemia, however, the 

 leg-bearing somites are 19 in number, and the 20th and 21st 

 somites form the genital segment in the male sex. In the female 

 all the apodous segments are coalesced. In the Cladocera the 

 female genital apertures are lateral or dorsal in position on the 

 posterior apodous division of the body. The male apertures are 

 lateral or ventral and often placed farther back, sometimes quite 

 at the end of the body. 



The genital openings are generally paired, but the female 

 opening is unpaired in the Anostraca, where the oviducts unite to 

 form a uterine chamber (Fig. 15, A, ut) with groups of gland- 

 cells on its walls. In some Cladocera the male opening is un- 

 paired. The terminal part of the vasa dcferentia forms a paired 

 eversible intromittent organ in the Anostraca (Fig. 15, B, p) and 

 in some Cladocera (Sididae). 



The spermatozoa are immobile and usually spherical. In the 

 Cladocera, however, they present a remarkable variety of form, 

 differing greatly sometimes even in the species of one genus. 



In the Anostraca the eggs are retained, sometimes till they 

 hatch, in the uterine portion of the oviduct. In the other Branchio- 

 poda they are carried after extrusion either in special receptacles 

 formed by the llth pair of trunk-limbs (Notostraca) or enclosed 

 within the valves of the shell (Conchostraca and Cladocera). In 

 the Conchostraca the egg-masses are attached to certain pairs 

 of specially modified trunk-limbs. In the Cladocera a special 

 brood-chamber is formed between the dorsal surface of the body 

 and the shell, and is closed behind by fleshy folds or prominences. 

 Further, in some Cladocera, if not all, a modification of the hypo- 

 dermis of the dorsal surface takes place for the secretion of a 

 nutritive fluid by which the embryos are nourished within the 

 brood-chamber. In the Cladocera the sexually produced " resting "- 

 or " winter "-eggs are deposited within the cast-off shell, and in 

 many a part of the shell becomes thickened and indurated, and 



