CH. VIIl] FROGS, TOADS, AND NEWTS 291 



makes its way into the pharynx, and thence passes along 

 the alimentary canal to the urinary bladder, becoming 

 sexually mature when three years old. If, however, the 

 creature gets attached to some highly vascular, and there- 

 fore nutritive, tissue of the frog, maturity may be attained 

 in five weeks. In such case self-fertilisation is effected 

 and the parent dies immediately after the discharge of 

 the eggs. 



Another Trematode that may be met with in the frog 

 is Ainphistoma subclavatum. The early stages of this 

 worm are passed in the water-snail Planorbis contortus. 

 On escaping from the first host the miniature form encysts 

 on the skin of a frog. Now frogs from time to time shed 

 their skin and swallow the cast-off pellicle, stuffing it 

 into their mouths by means of the front feet. Thanks 

 to this habit on the part of the host the encysted Amphi- 

 stoma reaches the stomach of the frog and there becomes 

 sexually mature. 



In the lungs and in the rectum there occurs commonly 

 Rhcibdonemcb nigrovenosum, a Nematode or Thread-worm. 

 This parasite is free-living during part of its existence, 

 but becomes sexually mature in the excrement of the frog. 

 The sexes pair and the fertilised ova produce embryos 

 which hatch within the body of the parent and devour 

 her prior to their escape. They then live in water or 

 may burrow into the tissues of small snails, but in neither 

 case do they become sexually mature unless swallowed 

 by a frog. They then make their way to the lungs and 

 obtain a length of as much as an inch. The generation 

 found in the lungs is hermaphrodite (successively), func- 



192 



