15 203 



laid their eggs, are often after the sexual periods so to speak dark spotted with 

 resting eggs. 



Only for some of the Rotifers from perennial ponds f. i. Triphylus hiciislris, 

 Asplanchnopus myrmeleo did the resting eggs appear and clearly showed that the 

 males, in spite of the utmost care, had escaped observation. In connection with this 

 fact, it must be remembered that just because so very many of these Rotifers in my 

 area of investigation developed their maxima simultaneously, it was necessary in 

 the very saine weeks, in the time from about 15 May to 15 June, to work at the 

 highest pressure possible. What could not be managed in this time, could not be 

 carried out before the next year. When that year arrived, fresh investigations were 

 carried out upon those Rotifers in which the males had escaped my attention then, 

 but it often happened that, in these very localities, just these species did not appear 

 that year. In this waj', it happened that the males of Eosphora digitata, Triphijlus 

 lacustris, several Noiommatidœ and others could not be procured. 



That the males of all these Rotifers from perennial ponds are able to 

 escape the investigation is due not onlj' to their presumed rarity, but also to several 

 other facts. Even if they are in some degree attracted by the light, this is not so 

 much the case as with the plancton Rotifers; all the specimens do not seek the 

 lighted edges of the vessels, very many of them never leave the algæ coatings upon 

 which they are born, and which cover the whole surface of the vessel, this makes 

 it much more difficult to And the males of these species than of the plancton Ro- 

 tifers, of which both sexes immediately dart towards the light source. Rotifers from out- 

 drying ponds especially Hydatina senta are only in a very slight degree attracted by light. 



Another factor which makes it difficult to find the males of very many pond 

 Rotifers is that the males in many of them are almost of the same structure as the 

 females; they are certainl}' smaller but not smaller than young newly born females. 

 Whereas a male of a plancton Rotifer, owing to its extremely small size, its pecu- 

 liar shape, and the enormous speed with which it moves, is easily recognised, the 

 slowly moving males of many of the pond Rotifers, shaped like young females, ai'e 

 very difficult to interpret for what they really are; much time is therefore wasted 

 in bringing presumed males, which are really onl}' young females, under high power. 



If however it is correct that many of these pond Rotifers attain huge maxima, with- 

 out any sexual period as the result, the question is, how this presumed fact may be 

 understood. — I am here inclined to suppose that, in all species, the huge maxima 

 inaugurate a sexual period, but that external factors, climatic conditions, prevent 

 its further development. The investigation has shown that, owing to sudden heat, 

 and especially owdng to sudden and very heavy rain, the large maxima may al- 

 most immediately be arrested. It seems, as if a strong and sudden dilution of the 

 water is able to kill the specimens in the course of very few hours; the eggs which 

 these specimens have laid, are then developed, but the external conditions, under 

 which this new brood is to live, do not allow of the rapid development of the sexual 

 products which is the condition of the development of the great maxima. 



