44 NATURAL SCIENCE [July 



While in the great majority of these cases the male differs from 

 the female not only by his much smaller size but also in the want 

 of an alimentary canal and of the characteristic rotiferan mastax, 

 four species are now known where this difference in structure 

 between the two sexes does not exist. In the very aberrant 

 genera Seison (9) and Parascison (14), which live as ectoparasites 

 on Nebalia, the male differs from the female only in the reproductive 

 organs. To these have recently been added the cases of Bhinops 

 vitrca (Rousselet 27) and Notommata wernecki (Rothert 26, 

 Rousselet 28), the latter being also a parasitic form living in 

 curious gall-like excrescences on the alga Vaucheria. In the last- 

 named rotifer the males are only to be distinguished from young- 

 females by very careful examination. While males are now known in 

 very many genera belonging to nearly all the families of the Rotifera, 

 a notable exception occurs in the case of the family Philodinidae, 

 of which, as yet, only females are known. Mr Rousselet (27) 

 suggests that possibly, as in the case of Notommata ivemechi, the 

 males of this family may have been overlooked from their resem- 

 blance to the females. He notes that ' resting-eggs ' (the association 

 of which with the occurrence of males will be referred to below) 

 have been identified with more or less certainty by Janson (22) and 

 Bryce in one or two species of Callidina. It must be remembered 

 however that the Philodinidae have developed to the highest degree 

 a method of resisting drought alternative to that afforded by the 

 resting-eggs, namely, the encystment of the adult animals, and also 

 that exclusively parthenogenetic reproduction is not unknown in 

 other groups of animals (e.g., Ostracoda and Cladocera). It is, 

 therefore, by no means impossible that the male sex may really 

 be non-existent in many of the species composing this family. Of 

 the difficulty attending the search for male rotifers we have a 

 striking example in the case of Stephanoceros, where the recent 

 discovery of the male (25) followed at an interval of not less 

 than 130 years that of the familiar and conspicuous female. 



The great activity of the male rotifers as compared with the 

 females is a characteristic feature, not only in those forms where the 

 females are sedentary (Rhizota), but even in the case of the most 

 powerful swimmers among the Ploima. In Brachionus, Hudson and 

 Gosse (13) describe the male as leading " a brief life of restless 

 energy, now darting from place to place so swiftly that the eye can 

 scarcely follow it, and now whirling round as if anchored by its 

 curved foot and penis. It often circles round the female, attaching 

 itself now here, now there, and forcing its companion to waltz round 

 and round with it, from the top of the phial to the bottom." In 

 these circumstances it is a matter of great difficulty to observe the 

 actual process of coition. Brightwell only speaks of seeing the 



