204 16 



Even if many maxima seem to be arrested without any pronounced sexual period, 

 a closer study of the colony will show that a sexual period has really been intended. 

 In the sexual periods the ovary commonly gets darker, and often changes its co- 

 lour from yellow to blue; this change in colour characteristic of impregnated male 

 producers maj' take place in a colony almost simultaneously in very manj' of the 

 females and rather suddenly, often in the course of only a few days. 



Almost simultaneously with the change of colour, the males appear; in a few 

 cases I have, during large maxima seen this alteration in colour of the Ovaria ; then 

 the maxima were suddenlj' brought to cessation, and no resting eggs were observed. 



A more thorough investigation will perhaps further show that we may also 

 in other ways calculate beforehand when the sexual periods will occur. Even if 

 this must be mainlj' regarded as music of the future some few observations 

 seem already now to make the following worth mentioning. 



Maupas and later on especially American authors have made it highly pro- 

 bable that, at all events in Hydatina senta, we possess two sorts of females, the 

 female producers and the male producers; the first-named produce onlj' 

 eggs from which females appear, the others eggs which, when unfertilised, give 

 rise to males, when fertilised, to resting eggs. In how far these observations may 

 be brought to bear upon all Rotifera, we do not know. For the present we 

 only know, that in outer form and in general anatomical structure the two sorts 

 of females cannot be distinguished from each other. — There is however the pos- 

 sibility, that more thorough investigations will find out these differences and fur- 

 thermore, that they will perhaps prove much more conspicuous in other species 

 than in Hijdatina. — For several of the plancton Rotifers it maj' often be shown 

 that the specimens which carrj' the small male eggs, are of a smaller size than 

 those which carrj' the fewer and much larger female eggs. This may be even more 

 conspicuously observed in Polyarthra platijptera, where the male producers are often 

 only half the size of the female producers, and have a much more pointed form. 

 Already Hudson-Gosse have observed this; in Tab. XIII fig. 5, 5 b the two sorts 

 of females are clearly figured. As however the females that carry the resting eggs, 

 are larger than those that carry the male eggs, and have not the peculiar acum- 

 inated form, it is probable that the acumination of the body in the male egg car- 

 rjdng females, is only a juvenile character which is obliterated during growth. In 

 some ponds I have seen Triavthra mystacina suddenlj' disappear at the moment 

 when the male producers appeared, and be replaced by T. longiseta, which at that 

 time was male egg carrying. 



As I have never been able to keep T. longiseta in the aquaria for more than 

 a few days, I have not been able to pursue the observations more thoroughly. 



At all events, after we have learned that the female sex in the Rotifera is most 

 probably divided into two forms: female and male producers, it is a natural course 

 to enquire, whether there might not, here as in the case of the Aphids, possibly be 

 some difference in the outer form of the two sorts of females, and all the more so 



