tion has begun on the fifth or sixth day. Our observations on the sex 

 system of D. paradoxum give a picture similar to the one described for 

 Polystoma integerrimum Froelich (see page 82 ). The wornnis examined 

 during the winter have a fully developed sex system, and in the spring 

 period it begins to act very quickly without any special period of acceler- 

 ated development of sex cells. The change to the active state takes place 

 much slower than in P. integerrimum , which is understandable when one 

 takes into consideration the gradualness in the increase of temperatures 

 in bodies of water where parasites and their hosts are located. The repro- 

 duction of D. paradoxum which begins in the spring continues almost the 

 entire sunnmer but the intensity of the deposition of eggs in May-June is 

 significantly higher than in the following months. The eggs of D. paradoxum 

 basically remain on the gills of the host, and Zeller observed up to 100 in 

 each fish with 3 mature parasites under artificial conditions. The larvae 

 emerge from the deposited eggs on the 12th to the 17th day according to p. 114 



Zeller (according to our data, 9-lOth day) and after a short time they in- 

 fect fish. The diporpa larvae which settle on the gills live for a certain 

 length of time singly and their growth develops rather slowly. After 

 reaching a determined stage of the development of the attaching apparatus 

 (2-3 pairs of clamps), a large part of the larvae ceases further growth if 

 it doesn't meet the same larvae with which it unites in pairs, grows to- 

 gether and continues mutual development. The larvae which remain single 

 live for a sufficiently long time, but all perish toward the winter. The 

 individuals which united in pairs begin to grow much more quickly than the single 

 ones and toward the spring of the following year they reach maturity. As 

 is apparent from what has been said before, the critical moment in the life 

 of D. paradoxum is the union of a pair of individuals, without which further 

 development is impossible. Taking into consideration that the periods of the 

 emergence of the larvae from the eggs are greatly extended one would expect 

 a large percentage of loss of some of the worms; however, this is net observed 

 in nature. The reasons for this appear to be a repeatedly noticed union of 

 the larvae of more or less different ages. Thus, relatively slow growth of 

 the larvae is an adaptive peculiarity to the singular life cycle of ^ paradoxum . 

 All in all, the life cycle of the latter (Fig. 122) has a number of primitive 

 traits characteristic of the forms with extended egg deposition, and just as many 

 highly specialized traits which are determined by the peculiarities of the 

 sex cycle. In addition to what has been said before, one must note still 

 another peculiarity in the nature of infection of the host: Younger 



hosts are infectod less than older. Thus, those of less than a year are 

 completely free of D. paradoxum, the yearlings either are completely vm- 

 infocted or infected exceptionally rarely and weakly and are often infected 

 not by pairs but by a single diporpa. How this phenomenon can be explained 

 one cannot say exactly; however, we are certain that here take place the 

 same circumstances about which we have often spoken earlier, that is, 

 different placts of location of older and younger fish. In connection with 

 this is the fact that the eggs of D^ paradoxum are found on the gills the fishes of 

 younger ages, which as a rule keep themselves separate from the older ones. 



118 



