VERMES. 47 



Order 3. Trcmatoda. Flukes. Parasites with mouth-opening placed 

 in an anterior sucker, forked or branched aproctous intestine, and supra- 

 pharyngeal ganglia. Usually provided with organs of adhesion in addi- 

 tion to the anterior sucker. Nearly all hermaphrodite [typical parts, 

 two testes with vasa deferentia, cirrus-sac with cirrus. Ovary (ger- 

 marium), two yolk-glands (vitellaria), oviduct, ootype with copulatory 

 passage (Laurer's canal), shell-glands]. Mostly oviparous. 



D i s t o m e sd endoparasites with not more than two suckers. Develop- 

 ment with metamorphosis and alternation of generations. [From the 

 egg an embryo, usually ciliated, is developed, generally in water, This 

 penetrates into an intermediate host (usually a mollusc), and develops 

 into a redia (with mouth and gut) or sporocyst (without gut), which in- 

 directly or directly produces cercariae (tailed, with forked intestine). 

 The cercaria, often after a second migration into another invertebrate host 

 (e.g., a water-snail), ultimately reaches the final host (a vertebrate) in 

 which the sexual form is attained]. Distqmum hepaticum (Fasciola 

 hepatica), liver-fluke. Body covered with chitinous prickles. In the bile- 

 ducts of the sheep and other domestic animals-, occasionally of man. 

 Ciliated aquatic embryo, with x-shaped eye spot ; the asexual nurse-forms 

 (sporocyst and redia) in Limnseus truncatulus. From the redia tailed 

 cercariae are developed, which become free and encyst on grass, &c., with 

 which they are swallowed by the vertebrate host. D. crassum, in the 

 intestine of the Chinese. D. haematobium (Bilharzia), bisexual ; in the 

 portal, mesenteric, and vesical veins of the Abyssinians. Monostomum 

 flavum, in water birds, the young form (Cercaria ephemera) in 

 Planorbis. 



Poly sto mere, mostly ectoparasites with more than two suckers. 

 Development direct, or without considerable metamorphosis. Poly- 

 Sternum integerrimum, in the urinary bladder of the frog. Egg pro- 

 duction commences in the spring (mutual fertilization), and the larva, 

 which is hatched in the water, enters the gill-chamber of a tadpole, loses 

 its five ciliated bands, and passing down the alimentary canal reaches 

 the bladder in eight weeks. Here it becomes sexually mature in three 

 or more years. (It rarely reaches the sexual stage in the gill-cavity, in 

 which case it dies after producing an egg.) Diplozqon paradoxum, on 

 the gills of many freshwater fish ; two separate animals (Diporpa) par- 

 tially fuse to a compound individual ; eggs with attachment threads ; 

 larva ciliated. Gyrodactylus. 



