PARASITES AND PREDATORS 59 



were also found at Green Street. Tadpoles from Hospital Pond had in 

 one year the spherical cysts of the nietacercariae of another treniatode, 

 either loose in the body cavity or attached to the mesenteries. 



There is, however, another parasite that deserves more attention, 

 because it is by far the largest, it is common, it has provided by its 

 investigation some unexpected light on the life liistory of the tadpole 

 itself, and it is of classical importance in parasitology. 



Polystoma integerrimum 



The material for this investigation (Savage, 1949) was the same 

 collection of tadpoles that served for the work on growth rates. It 

 was not a study of the parasite in isolation, but was analogous to the 

 reports of a Medical Officer of Health when he describes the course of 

 an epidemic of typhoid in his district. In such a case, he does not treat 

 the subject as a bacteriologist would, nor as a physician — it is the course 

 of the epidemic that takes his attention. The work now to be described 

 depends on the examination of about a thousand tadpoles, containing 

 about four thousand parasites. Eight epizootics in five ponds were 

 studied. 



The life history of this parasite was studied by Zeller (1872, 1876) 

 and by Gallien (1935). The eggs of the mature fluke, which inhabits 

 the bladder of the adult frog, are voided with the urine into the water 

 of the pond when the frogs are spawning. The eggs of the trematode 

 take longer to hatch than those of the frog, so that larval flukes find 

 the tadpoles already at the active stage of their lives. The larvae swim 

 rapidly by means of cilia, and enter the tadpoles by swimming or 

 creeping through the spiracle. Once in the gill chamber, they attach 

 themselves to the gills or to other parts and there develop in either of 

 two distinct ways. If the tadpole is fairly advanced in development, 

 the larvae change little although some differentiation does occur. At 

 the time of metamorphosis, they migrate through the alimentary 

 canal to the newly formed bladder of the young frog, and there reach 

 maturity in about three years. If, however, the tadpole is young, the 

 parasites may begin to grow so rapidly that changes are perceptible in 

 a few hours. They develop into neotenic larvae, smaller and struc- 

 turally different from the adults, but capable of laying self-fertilized 

 eggs after a few weeks of growth. These eggs are passed out of the 

 gill chamber into the pond and hatch after some weeks into larvae 

 identical in appearance to the larvae that were the progeny of the 



