and the latter from 10 to 20 days, and sometimes it is delayed for a certain 

 additional period. The development of eggs of P. integerrimum in natural 

 conditions is rather lengthy. Depending upon temperature conditions, it 

 lasts from 20 to 30 days in the Leningrad region and from 40 to 50 days in 

 France (according to Gallien). The free -swimming larvae emerging from 

 the eggs seek their hosts, which happen to be tadpoles which at that time 

 are undergoing different stages of development, starting with the appearance 

 of inner gills. The fate of the larvae becomes different after their attach- 

 ment on the gills of the tadpoles. If the larva attaches itself to the gills of 

 a very young tadpole it begins to feed intensively and grow fast, changing 

 into a special mature form, the so-called "gill" P. integerriinum. In the 

 case when the larva falls on more mature tadpoles they feed less intensively 

 and grow more slowly without the development of the sex system (their fate 

 will be described somewhat later). The "gill" P. integerrimum reach full 

 maturity 20 to 25 days after the settling of the larvae on the gills of the 

 tadpole and from that time begin to produce eggs. The gill form is very 

 interesting in structure because it differs significantly from P. integerrimum 

 from the urinary bladder. The gill P. integerrimum (Fig. 127) has a less 

 distinct, so to speak, broadened configuration of the body and the attaching 

 disc is not delineated from the body, the interior organization is also 

 sharply distinct. The intestinal tract has a small number of lateral growths 

 and interior anastomoses and the location of both is less regular than among 

 P. integerrimum from the urinary bladder. There are especially significant 

 differences in the structure of the sex system. The ovary of the gill form 

 is very long and almost straight with a flask-like widening at the anterior 

 end filled with rapidly developing obgonial cells. Oocytes in the ovary lie 

 one after another in its longer part and are efferred (expelled, nobis) from 

 the ovary according to their degree of maturity. Generally the shape of 

 the ovary of the "giU" P. integerrimum resembles thc-t of the young, still 

 immature P. integerrimum of the urinary bladder (Fig. 128). Among the 

 gill forms the vaginal ducts are completely absent and actually, so to 

 speak, there is no uterus as among many of the lowest monogenetic 

 trematodes, but only an ootype in which a single egg is formed at a time P- 124 



which is immediately carried outside. According to the data of Gallien^ 

 the ductus genito-intestinalis is absent among the gill type of the parasite, 

 and apparently it is really so. The male sex system is represented by a 

 round testis and a seminal duct which opens into the copulatory organ of 

 identical structure to the one of P. integerrimum from the urinary bladaer. 

 According to Gallien, the seminal duct is absent and the copulatory organ 

 is not connected in any way with the testis, so that it represents, in such a 

 fashion, only a rudimentary remnant. According to Gallien, fertilization 

 takes place through a special duct uniting the male and female sex systems. 

 In his time Zeller also wrote about this duct. Our observations do not 

 substantiate these data. Fertilization of gill forms takes place just as 

 among lowest monogenetic trematodes --not through a special canal, but 

 through the ootype. The life span of the gill form coincides with the 

 period of existence of the inner gills of the tadpole --with their disap- 

 pearance, the "gill" P. integer rinnum perishes. The deposition of eggs 



127 



