v PHYLUM PLATYHELMINTHES 273 



on the surface independently of the uterus : internally it com- 

 municates with the oviduct through the main vitelline duct. 

 In some of the Heterocotylea there is a genito-intestinal canal 

 occupying a corresponding position to Laurer's canal, but opening 

 into the intestine. In the Aspidocotylca this is replaced by a 

 stalked yolk-receptacle. There is nearly always a genital atrium 

 common to the ducts of both sexes. 



In the Temnocephalea (Figs. 201, 202) there is a genital atrium 

 and a single genital aperture. Thare are two pairs of compact 

 testes ; the right and left vasa deferentia unite in a vesicula semi- 

 nalis, and granule or prostate glands are well developed. The 

 cirrus has a chitinous tube and a variety of eversible spines. There 

 is a single compact ovary ; the oviduct has connected with it a 

 large receptaculum, and dilates posteriorly to form an ootype into 

 which the shell-glands open. Actinodactylclla (Fig. 202) alone 

 has a bursa copulatrix (b. c.}. 



In the ordinary Cestodes each segment or proglottis contains a 

 set of reproductive organs similar to those of a Trernatode. There 

 may be a single genital aperture leading into a genital cloaca, into 

 which both male and female ducts open ; or the male and female 

 apertures may be distinct. The testis is divided into numerous 

 minute lobes, from which proceed a number of fine canals joining 

 together to form the vas deferens, at the extremity of which is the 

 chitinous cirrus. There are two germaria, and either a single 

 vitelline gland, or two. The oviduct has its origin in a sort of 

 isthmus connecting the two germaria. It receives a narrow 

 fertilising duct from the receptaculum seminis and the vitelline 

 ducts and becomes surrounded by a rounded mass of shell-glands 

 to form the ootype, which is not definitely enlarged. Further 

 forward it gives off the uterus. The latter is at first a simple 

 cylindrical outgrowth from the oviduct, but it usually becomes 

 large and may be extensively ramified. It has no external 

 opening in most instances, so that the eggs only escape from it by 

 the breaking down of the proglottis or by dehiscence. But in 

 some (c. ()., Dibnthriocephal'iis), it has an independent exter- 

 nal opening. The female aperture leads into a narrow canal 

 -the vagina which ends in a receptaculum seminis from which 

 the narrow fertilising duct conveys the sperms to the oviduct. 



The development of some of the Platyhelminthes (Rhabdoccela, 

 Monogenetic Trematodes) is direct i.e., not complicated by the 

 occurrence of a metamorphosis ; in the Digenetic Trematodes, the 

 Cestodes, and some of the Planarians a metamorphosis occurs. 



The eggs of the Polyclads, each of which consists merely of the 

 fertilised ovum (oosperm) usually enclosed in an egg-shell, are, 

 in most instances, laid in large numbers embedded in a plate of 

 slimy secretion. The ovum (Fig. 216) divides first into two 

 equal parts, then into four. From each of these four cells is then 



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