PHYLUM PLATYHELMINTHES 



229 



tcstes, vasa deferentia, and cirrus. The tcstes (to.) are two greatly 

 ramified tubes, which occupy the middle part of the body, one 

 situated behind the other. From each testis there runs forwards 

 a duct, the vas deferens, the two vasa deferentia (v.d.) opening 

 anteriorly into an elongated sac, the vesicula seminalis (v. s.), from 

 which a narrow tube the ejaculatory duct (Fig. 178 ej.cl.) leads 

 to the male aperture at the ex- 

 tremity of the penis. The female 

 part of the reproductive apparatus 

 consists of a single ovary, an ovi- 

 duct, a uterus, vitelline glands, vitel- 

 line ducts, and shell- glands. The 

 ovary (Fig. 177, ov.) is a branched 

 tube situated on the right-hand 

 side in front of the testes; the 

 branches open into a common 

 narrow tube, the oviduct (od.). The 

 vitelline glands (vit.} consist of very 

 numerous, minute, rounded folli- 

 cles, which occupy a considerable 

 zone in the lateral regions of the 

 body. On each side are two large 

 ducts, anterior and posterior, unit- 

 ing to form a single main lateral 

 duct, right or left ; and these run 

 nearly transversely inwards to open 

 into a small sac, the yolk reser- 

 voir. From this a single median 



vitelline duct runs backwards for a short distance to join the 

 oviduct. Around the j unction are grouped a mass of unicellular 

 shell-glands (sh. gl.), each of which is produced into a narrow 

 process or duct opening into the end of the oviduct or the be- 

 ginning of the uterus. The uterus (ut.) is a wide convoluted tube, 

 formed by the union of the oviduct and median vitelline duct ; 

 in front it opens close to the base of the penis. When the penis 

 is withdrawn, a small cavity, the genital sinus or cloaca, is formed, 

 common to the external apertures of both male and female ducts. 

 A canal, termed the canal of Laurer, leads from the junction of 

 the oviduct and median vitelline duct to open externally on the 

 dorsal surface. 



Development. Each ovum on impregnation bceomes sur- 

 rounded by a mass of vitelline matter or yolk derived from the 

 yolk-glands. It then becomes enclosed in a chitinous shell, the 

 substance of which is derived from the shell-glands. The com- 

 pleted egg remains for a little time in the uterus while the con- 

 tained ovum is undergoing the process of segmentation ; eventually 

 it is discharged, and passing down the bile-ducts of the Sheep 



FIG. 178. Distcmum hepaticum. 



Terminal part of the reproductive 

 apparatus. <;/. ejaculatory duct ; /'/ 

 female aperture ; g. unicellular 

 glands ; od. terminal part of oviduct ; 

 p. penis ; ps. penis sheath ; s. sucker ; 

 V. ii. vasa deferentia ; c. s. vesicula 

 seminalis. (After Sommer.) 



