9 H)7 



servations, carried out in twenty-two different smaller lakes and ponds and accom- 

 plished in 12 — 16 months with intervals of eight to fourteen days. 



I am inclined to think it a tacit supposition that the Rotifer life in ponds is 

 almost totally obliterated during the winter half-year, and that the many different 

 species all spend the winter as resting eggs; this is a very great mistake. Very many 

 .species, belonging to all the three above-named associations, may be found at tem- 

 peratures very near zero, and in those cases where I have been able to study the 

 life in ponds which have been iced, I have found at airtemperatures of ^ 10° C, 

 even if the pond was icebound for a month, almost quite the same pelagic Rotifer 

 fauna as before the pond was icecovered. 



The sexual periods manifest themselves not so much owing to the presence 

 of the males which, because they are so exceedingly small, are almost always over- 

 looked, but much more owing to the presence of the dark resting eggs which on 

 the one hand are the result of the pairing process, on the other hand cannot be 

 formed, if a pairing process does not take place, the presence of which therefore 

 indicates the point of time when the males have appeared. 



A long series of investigations carried on from this laboratory have shown that 

 in the different species of Rotifera the sexual periods differ very much with regard 

 to strength and distinctness. There are species which year after year have very 

 pronounced sexual periods simultaneously in very many localities and in which the 

 males in these sexual periods occur in millions, other species where the sexual 

 periods almost always and everywhere are very little pronounced, and which mainh' 

 seem to propagate parthenogenetically. In all these species the number of males 

 in a given locality is exceedingly small. Finally, there are species in which sexual periods 

 have never been observed, and which really seem to propagate parthenogenetically 

 the whole year round. The investigations have further shown, that just those 

 species in which pronounced sexual periods in very many localities appear almost 

 simultaneously, when appearing under special conditions, here seem to be almost 

 acyclic i, e. without pronounced sexual periods. This is the case with those species 

 which are not only to be found in the plancton of smaller ponds, but also occur 

 in the pelagic region in larger lakes. These species may in the ponds be di- or poly- 

 cyclic, whereas as plancton organisms in the larger lakes they are monocyclic, or 

 for several years even acyclic. 



It will be understood that, when hunting for Rotifer males, it is of the greatest 

 significance to know, when the sexual periods for the different species occur; then 

 only, in these often very sharply defined periods, the males occur. If however we 

 onh' use the resting eggs as indicators of a sexual period, we shall very often be 

 too late to get the males; the life of the males is often i-estricted to only a few 

 hours and is sure never to last more than four or five days; as these daj's for the 

 total amount of specimens of a given species almost coincide and are only rarely 

 distributed over more than about 10 days, and as further the resting eggs appear 

 in the last part of this period and "are often carried by the females for rather a 



U, K. 1). VidensU, Sclsk. SUr.. nnlurv. og niallieni. Afd., s. KielcUe. IV. :i. 27 



