v PHYLUM PLATYHELMINTHES 133 



vasa deferentia (v. d) opening anteriorly into an elongated 

 sac, the vesicula seminalis (v. s), from which a narrow tube — 

 the e/aculatory duct (ej) — leads to the male aperture at the 

 extremity of the cirrus. The female part of the apparatus 

 consists of a single ovary, an oviduct, a uterus, vitelline or 

 yolk-glands, vitelline ducts and shell-glands . The ovary (ov) 

 is a branched tube situated on the right side in front of the 

 testes : the branches open into a common duct, the oviduct 

 (od). The vitelline glands (vit) consist of very numerous 

 minute rounded follicles, which occupy a considerable zone 

 in the lateral regions of the body. The two main vitelline 

 ducts, right and left, run transversely inwards to open into a 

 small sac — the yolk reservoir. From this a single median 

 duct passes to join the oviduct. Around the junction is a 

 mass of unicellular shell-glands (sh.gt). The uterus (ut) is 

 a wide convoluted tube formed by the union of the median 

 vitelline duct and the oviduct. In front it opens close to 

 the base of the penis. A canal termed the canal of Laurer 

 leads from the junction of the oviduct and median vitel- 

 line duct to open externally on the dorsal surface of the 

 body. 



Each ovum on impregnation becomes surrounded 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 secretion of the 

 shell-glands. The completed egg remains for a time in the 

 uterus ; afterwards it is discharged, and, passing down 

 the bile-ducts of the sheep into the intestine, reaches the 

 exterior with the faeces. When it escapes from the egg, the 

 ciliated embryo, as it is termed (Fig. 70, A), has the form of 

 a somewhat conical body, covered all over with vibratile 

 cilia, and with two spots of pigment, the eye-spots, near the 

 broader or anterior end, which is provided with a triangular 



