402 



GENERAL ZOOLOGY 



Examples:' Tristoma coccineum, parasitic on the gills of the 

 swordfish; Poly stoma integerrimum, as a larva, parasitizes the 



gills of the frog tadpole, but when 

 the gills are absorbed at meta- 

 morphosis, it passes into the 

 pharynx, through the alimentary 

 canal to the urinary bladder, 

 where it is found in the adult 

 frog as an internal parasite. 

 Order 2. Digenea. Endoparasites 

 living in two or more hosts, to 

 which they attach themselves by 

 one or two median suckers, of 

 which the anterior one is the 

 mouth, and the second one, when 

 present, is for attachment only 

 (Fig. 229). 



Example : Fasciola hepatica, the 

 liver fluke, lives as an adult in the 

 liver of sheep, cattle, man, and 

 other animals, where sexual 

 reproduction takes place. The 

 young embryos pass down the 

 bile duct into the intestine, and 

 out of the sheep's body into 

 water, where they develop into a 

 ciliated miracidium larva. Its 

 intermediate host is a water snail 

 of the genus Limnaea, whose tis- 

 sues it enters by boring, and 

 inside of which it forms a sporo- 

 cyst. Eggs contained in the 

 sporocyst develop parthenoge- 

 netically into rediae, which leave 

 the sporocyst and enter the tis- 

 sues. A number of parthenoge- 

 netic redia generations may be 

 produced, followed, finally, by different larvae known as 

 cercariae. These resemble the adult, except that they have 

 a tail, which serves as an organ of locomotion until the animal 



Fig. 229. — Fasciola hepatica, dia- 

 grammatic from the ventral side. 

 The digestive tract in solid black is 

 shown only in the left of the figure. 

 The reproductive system is com- 

 plete only in the right side of the 

 figure, d, sperm duct; e, excre- 

 tory pore and a small portion of the 

 terminal excretory tubes; m, mouth; 

 o, ovary; p, penis; s, shell gland; 

 t, testis; u, uterus; v, vagina; y, 

 yolk glands. The ventral sucker 

 is indicated by a circle below the 

 penis. 



