gills by them, the larva of ^ integerrimum passes through the intestinal 

 tract to the urinary bladder where it finally settles. The last pair of the 

 suckers of the larva Fig. 206, C) develop in the urinary bladder and it 

 terminates its metamorphosis. The growth of the individual continues 

 during its entire life, i. e. , about 4 to 5 years, whereas maturity is 

 reached at the end of the second year. The attaching disc of Polystoma 

 of the urinary bladder is >srell-expres8ed and delineated from the rest of 



Fig. 206. 

 the larva. 



Polystoma integerrimum Froelich, stages of metamorphosis of 

 Explanation in text. (According to Zeller, 1872). 



the body from the very beginning of its development. Just as among the 

 gill form, the edge hooks do not grow, whereas the middle hooks reach 

 considerable sizes -forming both offshoots and apparently grow during the 

 greater part of the life of the individual (at any rate after it reaches ma- 

 turity). Suckers are formed around the same pairs of lateral hooks but 

 have a more regular shape and already at the end of the first year they 

 have the same sizes while continuing, at the same time, to grow for a 

 relatively long time. The intestinal tract acquires its final shape, with three 

 characteristic internal anastomoses by the end of the first year ot life. The 

 sex system differs from the one of the gill individuals by a more compact 

 structure of the ovary and the presence of the ductus genito-intestinalis, 

 2 vaginal ducts, an elongated uterus capable of holding more than two dozen 

 eggs, and with a large follicular testis. It is interesting that the structure 

 of the copulatory organ in both forms is identical; also identical in form 

 and number are the hooks which are located as a wreath at the base of the 

 penis. 



p. 186 



The development of P. nearcticum (Paul) was studied by the 

 author of the species (Paul, 1938) and it is very similar to the one of P. 

 integerrimum. It is interesting that the gill form is also discovered in this 



201 



