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chlanidœ, Aniiræadæ, in my aquaria. I have here never seen a trace of a sexual period. 

 In the summer half-j'ear tlie cultures of all these species have always died out in 

 the course of a few weeks, during the sexual periods in the course af a few days. 



The investigations in nature have further shown that very many of those Ro- 

 tifers which are realh' very common pond Rotifers belonging to the vegetation zone 

 and occurring in almost every pond f. i. Dinocharis pocilhim, Cathypna liina, Mono- 

 styla corniita, many of the Philodinidœ, Pterodina patina, Metopidia and many others, 

 hardly ever occur in those great maxima which are characteristic of many plancton 

 Rotifers, or for such species which are living in pools that are drying out (f. i. Hy- 

 tina senta). They are common the whole year round, perhaps a little more common 

 in summer than in winter, but all in all, great variation in number the whole year 

 round cannot be demonstrated. Even in these pond species, the males are either 

 totally unknown or onlj' observed once in a single specimen. If however these very 

 pond Rotifers are studied j'ear after j'ear and in very many localities, it happens, as 

 stated above, that the naturalist now and then comes across ponds, where now one 

 now another of the above-named specimens occurs in countless numbers, i. e. has 

 one of its great maxima. I have seen these great maxima in several of the Philo- 

 dinidœ, Pterodina patina, Metopidia lepadella, Monostyla cornuta, but only a few 

 times in the course of twenty years and onlj' in a single localitj\ Commonly these 

 great maxima occurred in the winter half-year. They were enormous immediately 

 before the ponds were frozen; they were observed under the ice during the first 

 weeks, but when the ponds thawed in April, they had disappeared again. These 

 species were cultivated in the laboratory during the maxima, the localities were visited 

 at intervals of only a few days, the results were always the same: not the slightest sign 

 of sexual periods : neither males nor resting eggs. Nevertheless the observation is 

 by no means without scientific value; more thorough investigations may perhaps 

 really find the males in other localities and fix the sexual periods. There is f. i. no 

 doubt that very many of the Philodinidœ have their greatest maxima in the winter 

 below the ice; if a sexual period should exist in this family too, and the males 

 most probable occur only sporadically and only at great intervals in the life history 

 of a colony, it is most probable that this would especially take place during the 

 winter half-j^ear. 



It may besides be emphasised, that it is very dangerous to conclude from 

 the failure to discover the sexual periods in a species (demonstration of resting 

 eggs and males), that this does not exist. Firstly it must be remembered that even 

 if a species in a given locality or a given latitude usuallj' onl}' propagates partheno 

 genetically, in other localities and other latitudes males maj^ occur and sexual periods 

 set in. This seems f. i. to be the rule with regard to many Ostracoda. the males of 

 which seem to be wholly absent already in our country, whereas they appear in 

 Rohemia and are common in Algeria. 



Experience further shows that even for the very same pond, in which the in- 

 vestigations are carried on, it is dangerous, on failing to ascertain a sexual period. 



