198 10 



long time, it ■«•ill be understood that the males have commonly disappeared when 

 a colony mainly carrying resting eggs has come under observation. Stress must 

 therefore be laid upon the point of tracing a sexual period before it comes. To do 

 this we possess different means. 



It has in an earlier paper (1898) been shown that, before a sexual period sets 

 in. the species in a given locaHty will very often increase enormously in number. 

 The species attains to what we commonly call its maximum. This observation has 

 been corroborated by almost all later authors. 



The magnitude of these maxima differs yeiy much from species to species; 

 for the same species they are neither of the same magnitude in different localities, 

 nor in different years in the same locaUty. In the same locality they maj' be dis- 

 placed a little from year to year, but all in all they occur at rather fixed seasons; 

 competition with other species, climatic conditions, variation in nourishment deter- 

 mine the magnitude. If the general conditions are of such a kind that they cannot 

 develop at the time which may be regarded as the natural one, the maxima as far 

 as my experience goes are not developed that year; a compensation later on in the 

 year very rarely takes place. 



The maxima are almost always greatest and most conspicuous in the "plancton 

 society". They may be so large, that the said Rotifers determine the colour of 

 the water. This f. i. was the case with some of the ponds, where Asplanchna prio- 

 donta had its great maximum; the water was then of a milky appearance; this is 

 however rather a rare case; it is commonly the colour of the chromatophores of 

 the food algæ which determines the colour of the pondwater. Especially in spring 

 the maxima in the pelagic pond Rotifers, owing to enormous parthenogenetic pro- 

 pagation, may set in with an almost incredible force; the large females of S. pec- 

 tinata may have seven or eight eggs in the oviduct; Rhinops and Asplanchna eight 

 or ten young ones and the young ones, before they are hatched, may have half 

 developed young ones in their oviducts. The duration of the maxima differs very 

 much; for some species it is several weeks, for othei's onh' six or seven days; they 

 may suddenly be cut off either because of the setting in of a sexual period, or owing 

 to unfavourable climatic conditions. Now and then two or more species have their 

 maxima simultaneously, but commonly they succeed each other; regular observa- 

 tions carried on for years show clearly that the succession, in which they appear 

 in the same pond, is almost the same year after year; most probably the species, 

 with regard to rapid propagation upon which the great maxima of course depend, 

 are bound to the great regular variations in climatic conditions upon which again 

 the amount of nourishment for the Rotifera depends. 



With regard to the creeping or slowly swimming Rotifer association, 

 whose habitat is the vegetation zone of perennial ponds, the maxima, according to my 

 experience, are by no means so great; they do not appear with that wonderful re- 

 gularity which is so characteristic of the plancton Rotifers. Whereas for very inany 

 of my ponds I am able to say in what week e'xactly the maximum of that plancton 



