40 THE BIOLOGY OF THE FROG chap. 



Parasites. — The frog, like most of the higher animals, 

 serves as the host of a large number of parasitic forms, be- 

 longing both to the animal and the vegetable kingdoms. 

 The leeches mentioned in the previous section might almost 

 be said to be parasitic, since they remain attached to the 

 frog for a long period. The larvae of blowflies (Calliphora, 

 Lucilia) sometimes infest the intestine of frogs ; but they 

 usually prove a greater pest to toads. The female lays its 

 eggs in the nostrils of the toad, and the larvse that hatch out 

 feed upon the membranes of the nasal cavity, and may 

 work their way into the brain and sometimes the eyes of 

 their host. I have found no record of their occurrence in 

 the nasal cavities of frogs, although it is not improbable that 

 they are occasionally found there. 



Of the several species of Nematodes found in the frog, 

 Rhabdonema nigrovenosa, which occurs in several European 

 species, is, perhaps, the best known, since its life history 

 presents several exceptional and interesting features. A 

 kind of alternation of generations occurs in this species, 

 there being a free form living outside the body, and a para- 

 sitic form which is usually found in the lungs. The latter is 

 hermaphroditic, and produces eggs which give rise to rhab- 

 ditiform embryos which pass into the alimentary canal and 

 thence outside the body. These embryos develop into the 

 free form, which consists of both males and females. The 

 eggs produced by the female are fertilized and develop 

 within her body. Here the embryos live and grow by 

 devouring the internal organs of their mother, after which 

 the young matricides make their escape into the water. 

 When opportunity offers, they crawl into the lungs of a frog, 

 and there develop into the parasitic hermaphroditic form. 

 An allied species, Ascaris entomelas Leidy, occurs in this 

 country in the lungs of Rana pipiens. 



