Form of Freshwater Coalenterate. Ill 



captured ones. 4. Light-coloured ovaries (ova with light- 

 coloured yolk) are more subject to the infection (the colour of 

 the yolk depends not so much upon the age as upon individual 

 peculiarities of the Sterlet) ; in yolks containing much fat 

 and of a yellowish-red colour the parasites live only for a 

 short time and then succumb. 5. Those fishes which are 

 brought from lower down the Volga are more infected than 

 those of this part [Kasan] . The disease prevails most strongly 

 during four or five months, from August to January, and 

 perhaps also rather longer, as in May still young secondary 

 buds have been found along with perfectly developed larvae. 



No external symptoms of disease are perceptible in the 

 Sterlets ; and even an infected roe is in no way distinguished 

 from the normal one upon a superficial examination. This is 

 due to the fact that our parasite lives in the interior of the 

 ova, and at the commencement of tliis phase of development 

 remains nearly motionless. The picture changes, however, 

 after the ovary has arrived at full maturity, and the com- 

 pletion of this developmental stage of the parasite, both 

 happening in most cases at the same time, the end of April 

 and in May ; the chorion of many infected ova, stretched by 

 the parasite, which is at this time full-grown, bursts prema- 

 turely, i. e. before the Sterlet has sjmwned, and the roe is 

 then in places traversed by a whitish slime, consisting of dead 

 and partially macerated Polypodia^ for the latter n)ust pass 

 directly from the ruptured chorion into fresh water in order 

 that they may continue to thrive. Infected ova of 3-4 millim. 

 in diameter * are at first in no way distinguished from healthy 

 ova either in the structure of their envelopes or in the nature 

 of the yolk, or in their relation to the blood-vessels. They 

 differ from the healthy ova by their diameter being 1-2 

 millim. greater, and on careful examination strike one by the 

 presence of a spirally-running band with undulated edges 

 over the whole surface under the envelopes, through which it 

 shows milk-white (PI. IV. figs. 1, 2). By this means the whole 

 structure, to compare it with something, reminds one of a 

 marble Easter-egg. With an advanced development of the 

 parasite the colour of the yolk changes to dark brown, which 

 is caused by the intermixture of minute granules, secretion- 

 products of the ectodermal cells of the parasite. 



The youngest stage observed by me in the development of 

 the parasitic form (A) of Polypodium hydriforme constituted a 



* I have found no parasites in smaller, and therefore younger, ova. It 

 is to be supposed that the parasites in the young stage live free and only 

 get into the ova of the Sterlet long after these have begun their develop- 

 ment. 



