FLUKES AS PARASITES 



201 



minute three -suckered fluke (Udonella caligoruin], numbers of 

 which attach themselves to the egg-bags of a degenerate crus- 

 tacean (Caligus\ living as a parasite upon the gills of the hake. 



Aquatic Amphibians do not escape from the attacks of Flukes, 

 a notable instance being afforded by one of these creatures (Poly- 

 stomum integerrimuni} which lives, when adult, in the urinary 

 bladder of the frog, and illustrates the transition from external 

 to internal parasitism. It adheres to the lining 

 of the bladder by means of a rounded projec- 

 tion at its hinder end, on which are situated six 

 suckers and many small hooks. The numerous 

 eggs are laid in spring, and pass from the frog's 

 body to the exterior, where they hatch out into 

 minute ciliated larvae, which actively swim about 

 in search of tadpoles. To understand what 

 happens next, it must be remembered that at 

 a certain stage in development a fold grows 

 back from the head of a tadpole, covering the 

 gill-slits, and uniting with the adjacent skin so 

 as to enclose a gill -chamber opening to the 

 exterior by a small hole or spiracle on the left 

 side. The continued existence of the fluke- 

 larva depends upon its finding a tadpole within 

 twenty-four hours, preferably one in the stage 

 described. If successful in this quest it swims 

 into the gill-chamber through the spiracle, and 

 becomes parasitic upon the gills. After living 

 for two months or so in these comfortable 

 quarters a change of residence becomes neces- 

 sary, for the tadpole is becoming a frog, the gills are disappearing, 

 and the gill-slits are closing up. The larva now makes its way 

 into the pharynx of its host, and passing through gullet, stomach, 

 and intestines, reaches and enters the bladder, where it becomes 

 adult in about three years. 



Mention must here be made of a singular species of many- 

 suckered Fluke (Diplozoon paradoxum, fig. 1 1 50) which lays its 

 eggs upon the gills of the minnow. Minute ciliated larvae hatch 

 out, which perish in from five to six hours unless they find another 

 host of the same kind. In that case, after further growth, they 

 fuse together in pairs, and become X- shaped adults, capable of 



Fig. 1149. Eight- suckered 

 Pollack - Fluke (Octobothrium 

 pollachii), enlarged 



