7 195 



have been observed at an earlier date, but are often mentioned in the literalure 

 only in few words and are very insufficiently drawn. 



It would by no means have been difficult to increase the number of the now 

 published males considerably; in all those cases where I have not been fairly sure 

 of the determination, or where I have been unable to distinguish the males from 

 other described males, they have been omitted. This has especially been the case 

 with the males of the fani. Notoinmatidce, Flosciilariadce, SynchœtaJœ and Bra- 

 chionidce. 



In the present stage of knowledge we still lack every acquaintance with the 

 males of the families Philodinidœ, Adinetadœ, Microcodidœ, Cathypnadœ and Anapodidœ. 

 Simultaneously with the preliminary investigations, a long series of papers appeared 

 which were the results of very many exhaustive and excellent studies, relating to 

 sex determination, mainly studied in Rotifera. Even if the chief object of these in- 

 vestigations was by no means to elucidate the cyclic propagation of the Rotifera, they 

 gave a long series of contributions which were of the greatest significance to the 

 elucidation of all problems relating to this difficult matter; most of these investigations 

 were carried out in North America. First it troubled me greatly that all the results, 

 gained in this way, were in the most striking contradiction to all that I saw; and 

 for years I saw no means of bringing the results from the laboratory investigations 

 in harmony with those carried out in Nature. The laboratory investigations were 

 almost all carried on upon Hydatina senta, partly upon Asplanchna and Proales, and 

 it was only after I had found in Nature the natural conditions under which these 

 species lived, and regular observations had begun here, that it was possible for me 

 to understand how the differences were to be interpreted. 



In this first part of the work I only wish to describe and figure the males, 

 give a general sketch of their anatomy, and contribute to the understanding of the 

 phenomenon, unique in the animal kingdom, that the one sex of the species in a 

 whole group of animals, counting more than 1000 species, all without any parasitic 

 phenomena, and living side by side with the other sex, in some cases is subject 

 to such an enormous reduction, that it is merely reduced to swimming testes "peram- 

 bulating bags of spermatozoa" (Rousselet 1897 a, p. 6), surrounded by a thin cuti- 

 cula, on its anterior part covered with a bunch of cilia; we have here to do with 

 freeswimming metazoa with the number of cells most probably smaller than in any 

 other organism in the animal kingdom, with a size not greater than a few times 

 that of a blood corpuscle, which pair at the very moment of birth and most probably 

 in some species die in almost the same hour in which they are born. 



Before entering upon the description of the males, it will be necessary to give 

 a preliminary sketch of the localities from which I have gathered the material; 

 the results with regard to the sexual periods, as they manifest themselves in na- 

 ture under natural conditions; further of the methods I have used for the study 

 of the males and lastly, a short view of my systematic conception of the whole 

 group. 



