558 



TREMATODA 



or short, pear-shaped or Y-shaped, generally opens posteriorly, but may 

 open dorsally above the acetabulum. 



The sexes are but rarely separate, hermaphroditism being usual. The 

 male organs consist of testes, which may be simple or branched, and are, 

 as a rule, situated posteriorly. The vas deferens leads forwards sometimes 

 through a vesicula seminalis to the genital opening, below which a cirrus 

 enclosed in a muscular pouch provided with glands, called the ' prostate,' 

 may be found. 



The female organs consist of an ovary, which may be branched and is 

 usually situated in front of the testes, and an ovarian duct, which joins with 

 the vitellarian duct from the yolk glands, making a tube, called the ' ootype,' 

 surrounded by the shell gland, in which the egg is formed (Fig. 210). 



A curious little canal, Laurer's canal, joins by its inner aperture the ootype 

 near the uterine tube, while its outer aperture is found on the dorsal surface. 

 The homology of this little canal is not quite clear ; it may be a vagina. Some- 

 times a receptaculum seminis is present. The ootype opens into the uterine 

 tube, which is usually much coiled, and has its anterior portion thickened, to 

 form the metatrema or vulva, which opens into the genital orifice (Fig. 211). 



The genital pore varies in position, being situated in the mid-line in front 

 of or behind the ventral sucker as a rule. Rarely it opens rather laterally 

 and has a muscvilar depression of its own. 



Life-History.— The full life-history of a number of forms has been 

 worked out by Leuckart and Thomas for Fasciola hepatica, under 

 which heading details will be given, by Looss for various amphi- 

 stomes, by Leiper for the genus Schistosoma, and by numerous 

 Japanese observers for other forms. 



Typically there is an alternation of generations where one sexual 

 generation is followed by two asexual generations. Two hosts are 

 required for the whole life-cycle. 



Leiper has given the following scheme for the development of a 

 digenetic trematode : — 



1. Definitive host . . . . . . . . Egg. 



2 . First transition . . . . . . . . Miracidium. 



[ Sporocyst 



^ Intermediate Sporocyst and daughter cysts 



host ' Sporocyst and Redise fCercaricB. 



Sporocyst, Rediae, and daugh- 

 { ter Rediae 



4. Second transition . . Free-swimming or encysted Cercarice. 



5. Definitive host Adults. 



Liihe has provided the following classification of Cercarise (sHghtly 

 modified for convenience of reference) : — 



A. Body without internal differentiation. With cuticular ala — 



LophocercaricB, 



B. Body with internal differentiation. Tails may or may not be 



forked : — 



I. Acetabulum absent — Monostomes. 

 11. Acetabulum present: — 



[a] Posteriorly situ3ite—Amphistomes. 



(b) Ventrally situate 



1. Mouth central — Gasterostomes'. 



2. Mouth terminal — Distomes. 



