ROTIFERA 213 



Two kinds of reproduction occur in the rotifers as in the clado- 

 ceran Crustacea, but in this case there are two kinds of females, 

 one of which always reproduces parthenogenetically, the eggs 

 developing to form females (female producers), while the other may 

 reproduce bisexually. In this second type (male producers) there are 

 eggs, often smaller than the female eggs, which develop quickly by 

 parthenogenesis into males. At various seasons after the appearance 

 of these male eggs there are produced by the same individual also 

 other eggs, distinguished by a thicker shell, and these have been 

 fertilized by the spermatozoa of the just hatched males injected 

 through the skin. These "resting" eggs are fertilized "male eggs" 

 and they only develop after a dormant period into females. 



The reproduction of a rotifer runs through a cycle in which at first 

 only parthenogenesis occurs but which is terminated by sexual re- 

 production. In rotifers which are typical members of freshwater 

 plankton, the cycles run to a time-table. There are "dicyclical" 

 rotifers like Asplatichna, which have two sexual periods, one in spring 

 and the other in autumn, while other forms like Pedalion are " mono- 

 cyclical ' ' and have only a sexual period in the autumn and pass the winter 

 as resting eggs. In rotifers like Hydatina, which inhabit puddles and 

 ponds, the sexual periods are very frequent and begin soon after 

 the resting eggs have hatched. The resting egg is a stage in which the 

 species can survive when the puddle dries up. Sexual reproduction 

 can be brought on in cultures by alteration of the external conditions. 



Besides the environmental types which have already been men- 

 tioned as free-swimming and inhabiting larger and smaller bodies of 

 water, the following rotifers may also be mentioned : 



Stephanoceros and Floscularia are sedentary forms which secrete 

 a protecting gelatinous tube into which they can withdraw rapidly. 

 Melicerta is another sedentary form which produces a tube formed 

 out of mud particles or its own faeces. 



Callidina and other genera are terrestrial forms which can remain 

 for a great part of the year in a dried-up condition but come to life 

 immediately when moistened by rain. Such forms are found, for 

 instance, in roof gutters and amongst moss. The group to which these 

 forms belong is called the "bdelloid" or leech-like rotifers, because 

 they not only swim, but progress by a looping method like that of 

 Hydra or a leech. 



