328 140 



As mentioned often in this work, the most primitive types of the Rotifera 

 like the Turbellaria live a creeping life either on the bottom or upon the vegetation. 

 In all these primitive Rotifers, the eggs are not carried by the mother animal but 

 laid upon a substratum. The diiference in size between the three kinds of eggs, 

 parthenogenetic female- and male-eggs, and fertilised resting eggs, is but small ; the 

 male eggs being most probably no more than one half shorter in diameter than 

 the female eggs (See Tab. IX, fig. 2 — 6). The males are but slightly reduced; com- 

 monly the alimentary canal is rudimentary, but all the other organs are present, 

 and even if the males are smaller than the females, they are of the same form 

 and often resemble the females in a high degree. 



The Rotifers which have diverged most from the primitive type, are the plane- 

 ton Rotifers, those which have totally emancipated themselves from a substratum, 

 living a pelagic life in the open water volumina, far away from a substratum of 

 any kind. These Rotifers in the main lack all areas of support for depositing the eggs; 

 commonly they carry the eggs themselves; a few of them either deposit them upon 

 other plancton organisms or bring forth living young. Now it is just among these 

 plancton Rotifers that the most reduced males occur. 



If we study the size of the three kinds of eggs in one of the plancton Rotifers, 

 it will be seen that there is here the greatest difference in size; especially the male 

 eggs are extremely small; further that whereas a female producer carries only. from 

 two to four eggs, the male producer carries about 12 — 16 eggs; if they carry resting 

 eggs their number is hardly ever more than one, rarely two. Moreover if we com- 

 pare the amount of the total yolk-mass which is to be found in the total lot of 

 male eggs with that which is found either in the female parthenogenetic eggs or in 

 the resting egg, it will be seen that in all three cases it is almost of equal size, 

 smallest perhaps in the resting egg (See Tab. XI, fig. 5 — 6). This in other words means 

 that the total amount of yolk-mass which is at disposal in the given moment of 

 egg-laying, is almost the same in all three cases. In one case the total amount is 

 used for a single egg, in the other for two or four, but when the males are to be 

 produced it is divided in no less than about sixteen different parts. The result is 

 that the amount of nutriment with which each male egg is provided, is only about 

 Vs to 'A of what a parthenogenetic female egg possesses. In accordance herewith the 

 males must at birth be smaller than the females. 



From these observations we are now able to understand the diminution in size 

 of the plancton rotifers, but of the most interesting fact, that the males are not only 

 reduced in size but also in their whole organisation, we have not hitherto got the 

 slightest explanation. For this we must take quite different phenomena into con- 

 sideration. 



It is a well-known fact that the eggs of the plancton organisms are commonly 

 verj' small; as often set forth this has most probably some connection with the fact 

 that the plancton organisms are mainly forced to carry the eggs and that therefore 

 the quantity of yolk-mass with which the eggs may be provided can only be relatively 



