the presence of tAvo different types of larvae which would develop from 

 eggs deposited by these different forms. However, actually the free- 

 swinnnraing larvae which have just emerged from the eggs of gill Polystoma 

 do not differ morphologically from those from the eggs of polystomes 

 which parasitize the urinary bladder. The eggs of P. integerrimum are 

 deposited in the water and the cleavage takes place already in the external 

 medium, among the individuals from the urinary bladder just as among 

 forms from the gill cavity. The free-swimming larva of P. integerrimum 



Fig. Z04. Polystoma integerrimum Froelich, the location of the ciliary 

 cells on the body of a free-swinnming larva. Left in ventral and right 

 in dorsal view. Impregnation with silver. Schematically. 



is more complexly organized than all the preceding forms, mainly by way 

 of the increase of the ciliary covering and its greater differentiation, and 

 of the separate parts of the body. The larva (Fig. 203) has an elongated 

 body shape which slightly narrows toward the anterior end, and also has 

 a well-expressed attaching disc. The latter is folded in the beginning and 

 then unfolds after a very short period following the emergence of the 

 larva from the egg. The sizes of the larvae are relatively large: the 

 length is about 0. 3 mm and the width is about 0. 07 - 0. 09 mm. The 

 ciliary covering is distributed in three zones (Fig. 204) just as among the 

 previous types; however, these zones have a more complex structure and 

 lie not only along the sides of the body but extend also to the dorsal and 

 ventral surfaces. For the study of details of the location of the ciliary- 

 covering of P. integerrimum we, together with T. Tsiborskayia, a 

 colleague of our laboratory, conducted a special research in which we 

 utilized the method of silvering widely accepted for the study of the 



p. 183 



198 



