420 PHYSIOLOGY OF THE INVERTEBRATA. 



The Trematoda are nearly always hermaphrodites ; and 

 the genital apparatus consists of the following parts: the 

 ovary, vitellarium, oviduct, uterus, vagina, common genital 

 vestibule, testis, vasa deferentia (internal and external), and 

 the penis and its sac. The ovum, as it passes into the oviduct, 

 is devoid of a vitelline membrane, and the vitellus or yolk is 

 clear ; but after fecundation, which takes place in the oviduct, 

 a shell is developed and the accessory yolk is added by the 

 action of the vitellarium. The oviduct, which is ciliated 

 internally, communicates with a duct which proceeds from 

 the testis; it also receives the vitellarian duct. The ovi- 

 duct then passes into the uterus, which terminates by the 

 vagina and common genital vestibule in close proximity to 

 the male organs. The oval-shaped testis, situated posteriorly 

 to the ovary, does not contain spermatozoa, but simply a 

 granular mass. The external vas deferens comes into contact 

 with the ovary, and then passes, after several bends, into 

 the anterior part of the body, terminating in the penis, 

 which occupies, in common with the uterus, the genital 

 vestibule. 



In the case of Aspidogaster (the above being an account 

 of its reproductive organs), the embryo assumes the adult 

 form without any metamorphosis ; but in other species, the 

 development is either direct* or accomplished by a com- 

 plicated metamorphosis,! accompanied by alternation of 

 generations. 



The ova of Aspidogaster, as they pass down the oviduct, 

 are impregnated, " either by the spermatozoa conveyed by 

 the internal vas deferens, or by those received by the vagina, 

 when copulation with another individual, or possibly self- 

 impregnation, occurs." 



As already stated, nearly all the Trematoda are herma- 

 phrodites, but among those that are dioecious is the parasitic 

 Bilharzia, which lives in the blood-vessels of man. The 

 female Mlharzia is much smaller than the male ; and a 



* Pclystomum, Oyrodactylus. f Distoma, Monoatomum. 



