LIFE-HISTORIES OF POLYSTOMATIDAE 



59 



At the time of the tadpole's metamorphosis, the young Polystomum 

 works its way down the pharynx into the oesophagus and along 

 the intestine, till it reaches and enters the opening of the bladder. 

 Three years afterwards it becomes mature. 



Sometimes, however, Polystomum experiences another fate. The 

 larvae settling down on the external gills of a young, recently- 

 hatched tadpole, and obtaining a 

 richer supply of blood than in the 

 previous case, grow far more rapidly, 

 so that in five weeks they are mature, 

 although still in the branchial 

 chamber of the tadpole. They do 

 not then wander into the aliment- 

 ary canal, but usually, having dis- 

 charged their eggs, die at the time 

 of the tadpole's metamorphosis. 

 Still more interesting, however, is 

 the difference between the genitalia 

 in these and in the normal Poly- 

 stomum. In contrast with the latter, 

 these possess (1) one testis and a 

 rudimentary penis ; and their sper- 

 matozoa differ in structure and 

 shape from those of the normal 

 Polystomum. (2) The vaginae are 

 absent, a fact connected with the 

 absence of a functional COpulatory FIG. 26. Polystmnum integerrim 



organ. (3) In compensation for 

 the loss of these, a duct connects 

 the single testis and the point of 

 union of oviduct and yolk-ducts, 

 and by this self-fertilisation occurs. 

 (4) The uterus is absent; the 

 " ootype " or duct into which the 

 shell -gland opens, communicating 

 directly with the exterior. In (1) and (4) these aberrant Poly- 

 stomum resemble P. ocellatum, from the Tortoise Emys europaea. 



Diplozoon paradoxum. The life -history of Diplozoon is 

 unique. For whereas the larvae of most animals grow up, 

 each into a single adult, in Diplozoon, of the few larvae that 



Two mature individuals in mutual 

 coition attached to the bladder 

 of a Frog, x 5. (After Zeller.) 

 d, Intestine ; ex.o, excretory pore, 

 dorsal in position, seen here by 

 transparency ; ey, eye-spots ; gl, 

 frontal glands ; m, mouth ; ph t 

 pharynx ; sd, adhering disc ; vaff, 

 vagina. 



