290 FROGS, TOADS, AND NEWTS [CH. VIII 



frogs that emerge from the water after the metamorphosis 

 it is not during the tadpole stages that the mortality is 

 heaviest. Internal parasites are fairly numerous. It is 

 from the rectum of the frog that the protozoa Nyctotherus 

 and Opalina are obtained. Both swim in the fluid 

 contents of the rectum, and live upon the residues of the 

 frog's food ; Nyctotherus has a mouth and presumably has 

 not been parasitic so long as Opalina, which has no aper- 

 ture into its body but absorbs liquid food through its 

 surface. This genus is multinucleate, and in reproduction 

 the animal divides up by repeated acts of fission into as 

 many individuals as there were nuclei at the first. The 

 minute forms which thus result pass out at the anus of 

 the frog into the water and obtain entry with the food 

 into the next generation of tadpoles. 



In the urinary bladder of the frog is frequently found 

 a Trematode worm, Polystomum integerrimum, easily 

 recognised by its crescent of six suckers round the 

 posterior end. The parent, after fertilisation, protrudes 

 her body through the urinary aperture and lays about 

 1000 eggs at the rate of 100 per diem. In some six 

 weeks a ciliated larva, about 0'3 mm. in length, emerges 

 and seeks, swimming by means of its cilia, a tadpole. 

 Failing to find this the larva dies in 24 hours. If the 

 quest be successful the young Polystomum enters the 

 gill-chamber of the tadpole via the opercular spout upon 

 the left side, and thereupon the cilia disappear. Some 

 eight or ten weeks are spent in the branchial chamber 

 and during this time the suckers of the adult worm are 

 developed. At the metamorphosis of the frog the parasite 



