ROTIFERA 241 



alimentary canal, but the ciliated disc, musculature and excretory- 

 system are well developed. Usually the male is not only smaller but 

 its ciliated disc and the alimentary canal are very much reduced and 

 the excretory system may be absent. The chief organ is the large 

 testis^ usually filled with ripe spermatozoa, which opens by a median 

 dorsal penis in many cases. Where the penis is absent the tapering 

 hinder end may be inserted in the cloaca of the female. Finally, it may 

 be mentioned that in one large family, the Philodinidae, which in- 

 cludes the genus Rotifer^ no male has ever been found. 



Two kinds of reproduction occur in the rotifers as in the cladoceran 

 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 Asplanchna, 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 passing 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 re- 

 production can be brought on in cultures by alteration of the external 

 conditions. 



Besides the environmental types which have been mentioned above 

 as free-swimming and inhabiting larger and smaller bodies of water, 

 the following rotifers may also be mentioned : 



Stephanoceros and Floscularia (Fig. 172 bis) are sedentary forms 

 which secrete a protecting gelatinous tube into which they can with- 

 draw 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 



