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Rotifer will begin, I am never able to say the same for the Rotifeis from Ihe vege- 

 tation zone. With regard to this association, year after year the investigalion offers 

 unsuspected surprises. Here, with regard to time as well as to quanlily the maxima 

 have an extremely casual character. To (his point we will shortly relurn. 



Ponds in which rare Rotifera have been found once, have been under regular 

 observation for years; these rare pond forms may in some months totally disappear; 

 in others year after year they always seem to be rare. To these very species belong 

 all those badly described males, of which the naturahst has only had the good 

 fortune to get a "glimpse", but not material enough for a more thorough study. 

 And even with regard to these species it certainly happens that exhaustive studies 

 in nature itself will now and then fall in a locality, where such a rare species really 

 has a great maximum with succeeding sexual periods and occurrence of males and 

 resting eggs. Elaborate studies year after year in the same locality and at the same 

 time and the same temperature will then very often result in the failure to fmd 

 the phenomenon again later on; the colony now year after year propagates only 

 parthenogenetically with abated propagation in winter and with a little quicker 

 propagation in the summer half-year. — This is f. i. the case with the Triphyliis 

 colonies, in North Sealand. 



With regard to the fixed Rotifer societies, those composed of the families 

 Melicertidce and Floscnlariadce, it might at a first glance seem difficult to speak ol 

 maxima at all. More thorough investigations from recent years have shown that 

 this is really allowable. Lacinularia socialis as well as different Melicerta-species 

 may, mainly at the highest summer temperatures, develop real carpets, with which 

 they coat the substratum upon which they are fastened. I shall return to this point 

 in the second part of this work. 



As soon as a maximum for a given species is observed in nature, the time 

 has arrived when it maj' be taken into the laboratory. Owing to the enormous amount 

 of specimens, pure cultures are easily procured. In very many cases the males then 

 appear in the cultures in the very week in which the material was taken in from na- 

 ture. The life time of these cultures is commonly very short, often only a few days, 

 but the time is long enough to procure the males. In the winter half-5'ear, when 

 no sexual period occurs, thej' may live for months. 



If we will try to found cultures from a few specimens of the same species and 

 at other times of the year, as far as my experience goes, it is quite impossible to 

 produce a maximum or a sexual period. Most probably, this is only possible for 

 the Rotifera of the pools that are drying out. At all events all these Rotifera, which 

 have been used for experimental work in the laboratory, and in which it has been 

 possible to develop sexual periods experimentally, belong to this category. It must 

 however be emphasised, that the researches, now published, have not been combined 

 with more thorough experimental work. It would be of the greatest interest if this 

 was one day carried out upon Rotifera belonging to localities which do not dry 

 out. In the winter half-year I have often had different species of Brachionidœ, Eu- 



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