202 14 



28 — 30° C. Then almost suddenly the rich microscopical life in the carpets disap- 

 peared. Quite the same result was gained the next j^ear, when heavj' and incessant 

 showers filled the pitholes and diluted the water. The algæ carpets did not disap- 

 pear, but during the rest of the year, they were not, or only very rarely, used. 



During the stajr of the aiiimals upon the carpets, the underside was used as a 

 localitj' in which the eggs were laid. As well known, the male eggs are commonly 

 smaller than the female eggs; simultaneously with the hatching of the eggmasses 

 on the algæ carpets in the vessels, we tried to find male eggs, and if possible isolate 

 them. The investigation showed, that just in these species, the difference in the 

 size of the two sorts of eggs is really but small, and that with regard to the length 

 of the diameter they overlap each other. 



Many of these pond Rotifers have been studied at regular very short intervals 

 in the very same ponds from the moment they appeared on the algæ carpets and 

 to the moment they again disappeared — the period commonly lasted three or four 

 weeks — ; fresh material was brought to the laboratory every second or third day. 

 Often great maxima were developed before the eyes of the observer; great quantities 

 of eggs were deposited in the vessels. The main task was to procure the males, and 

 in many cases the task was really accomplished. There is no doubt, that if it had 

 not been observed that these carpets were used as localities in which the eggs were 

 deposited, most of the males belonging to the creeping or slowly swimming Rotifers 

 would not have been found. 



I am inclined to suppose that the investigation, in the manner in which it is 

 carried on, is more likely to give an idea of the occurrence of the males under 

 natural conditions than previous investigations. The common impression is that the 

 males among these Rotifers from perennial ponds are extremely rare. Huge maxima may 

 reallj' set in, the ponds maj' teem with females, but the females are almost all 

 female producers, only very few male producers occur. When the specimens are 

 hatched in the vessels, most of them seek the lighted edge of the vessels; hour after 

 hour the edges are sucked clean by a pipette and the material brought under the 

 microscope; among many hundreds of females belonging to specimens of the common 

 pond rotifer life, one or two males appeared; during the whole period the number 

 was not augmented. Often the vessels have teemed with Dinocharis, Triphijlus, Ste- 

 phanops, Scandium, species of Diiirella, Metopidia, Cathi/pna, Monostgla, Rattulus, No- 

 tommata but of most of the species, e\en if they were studied in freshly gathered 

 material, collected at intervals of only a few days, it was impossible to get a single 

 male. Of some of the species f. i. Dinocharis and Scaridium longicaudiim Weber 

 observed the males in Switzerland. What especially favours the supposition that a 

 sexual period really does not appear in our latitudes, at all events not in all loca- 

 lities, and not every year, is the circumstance that of these species it has almost always 

 been impossible to get the resting eggs. Just upon this point there is the greatest diffe- 

 rence between the species from perennial ponds and those from pools drying out, where 

 the males are regularly found. The algæ carpets, upon which Hydatina colonies have 



