146 INVERTEBRATE MORPHOLOGY. 



to the exterior by two pores lying close together on the ventral 

 surface rather nearer the anterior than the posterior end. 

 The male apparatus consists in the Polystomece of numerous 

 closely-aggregated testes, or else, as in the Distomece (Fig. 76), 

 of only two situated in the posterior half of the body ; the 

 ducts from the testes pass forwards towards the genital pore, 

 near which they unite to form a sac-like seminal vesicle, 

 from whose anterior end the single vas deferens is continued 

 on towards the pore, passing in the latter part of its course 

 through a muscular protrusible intromittent organ, the cirrus. 

 The ovary is single, and its duct shortly after leaving it 

 receives the ducts coming from two yolk-glands situated one 

 on either side of the body, and is surrounded at about the 

 same region by a shell-gland, consisting of a number of uni- 

 cellular glands arranged in a radiating manner around the 

 oviduct. Beyond its union with these ducts the oviduct 

 either runs almost directly to the genital atrium, opening 

 into it in close proximity to the cirrus, or else pursues a 

 winding contorted course through the parenchyma and serves 

 as a iiterus or oott/p, within which the ova undergo a portion 

 of their development. 



From the oviduct in the region where the ducts from the vitellaria and 

 shell-gland open into it one or more canals may arise whose significance is to 

 a certain extent problematical. In the Distomese one such canal occurs, and 

 when a seminal receptacle is present it stands in more or less close relations 

 to this canal, known as Laurer's canal, which, after a short course, opens to 

 the exterior on the dorsal surface of the body. In some Polystomece two 

 canals arise from the yolk-ducts and pass forwards parallel to the uterus 

 to open by a number of pores situated on the margin of the body. These 

 canals have been termed the vagina, and in some forms are represented by 

 a single canal. In addition to the vagina, however, another canal is pres- 

 ent which has been shown in Polystomum and Spltyramira to open into 

 the digestive tract, and has been homologized with Laurer's canal of the 

 Distomese. 



It seems pretty certain that the vagina of the Polystomeas functions in 

 copulation, the genital orifice of one Polystomum having been observed to 

 come into contact with the vaginal openings of the other during that act. 

 But the Laurer canals do not seem to have any such function, and it has 

 been suggested that they may serve for the removal of surplus yolk- 

 material produced in accordance with the favorable conditions for nutri- 

 tion offered by the parasitic mode of existence of the Trematodes. 



