48 ESSENTIALS OF ZOOLOGY 



smaller than the lateral nerve pass backwards from the 

 pharyngeal commissure. 



The reproductive organs are highly developed and com- 

 plicated. Trematodes are hermaphrodite. The openings of 

 the male and female ducts to the exterior occur close together 

 between the two suckers. 



The male organs consist of two branched testes lying 

 near the middle of the body. They each give off a vas 

 deferens which extends forward to the neighbourhood of 

 the ventral sucker. There they run into a median sac, the 

 seminal vesicle. The seminal vesicle opens into the ejacu- 

 latory duct, the anterior end of which is invaginated to form 

 a penis or cirrus, which can be withdrawn to form a cirrus 

 sac or protruded beyond the male opening. 



The female organs are made up of a dendritic ovary, from 

 which issues the oviduct. Shortly after leaving the ovary 

 the oviduct receives the vitellarian duct formed by the union 

 of two ducts leading from the large yolk glands which lie 

 along the sides of the worm. At this point also a short duct 

 leads to the dorsal surface, where it opens by a pore. This 

 is Laurer's canal. This part of, the duct, moreover, is swollen 

 and beset by the mass of gland cells called the shell gland. 

 Beyond this region the oviduct becomes widened and con- 

 voluted, and ultimately ends near to the male opening. 



The spermatozoa formed in the testes are passed along 

 the vasa deferentia to the seminal vesicle and ejaculated by 

 the cirrus into the Laurer's canal of another individual. The 

 ova are fertilised, and during the time they are being sur- 

 rounded by a horny shell by the shell gland they receive 

 a quantum of yolk from the vitellarian duct. The eggs are 

 then passed into the convoluted part of the oviduct, often 

 called the uterus, and are shed through its opening into the 

 bile duct of the host. 



It is plain from the above and the figures, also by an 

 inspection of specimens and preparations, that the fluke body, 

 though thin, is occupied very fully by various ducts concerned 

 with digestion, with excretion and reproduction. The organs 

 are embedded in a mass of cells which extends from ectoderm 

 to ectoderm, and these cells are more or less compact, forming 



