5 193 



again later {Salpina, Coliinis, Dinocharis, Diglena, Scavidhim, HudnliiHi, Capvus, Cnliinis). 

 Almost simultaneously Rousselet's descriptions of Rhinops (1897 a) and of Ptcrndina 

 (1898) appeared. Very exhaustive descriptions and elaborate drawings are furnislicd 

 by MoNTGOiMERY of the males of Flosciilaria proboscidea (1903), by Hamiuiügek of 

 Lacinularia socialis (1907) by Krätschmar of Anarœa aciileata (1908) and by de Beau- 

 champ of Eosphora digifata (1905). In 1903, Rousselet (p. 172) gives a list of all known 

 Rotifer males, indicating more than 100. Very many of these males have however only 

 been observed, neither described nor drawn. Simultaneoush' with these papers others 

 appear in which males certainly are described bul often in such a manner that it 

 is almost impossible to recognise them; this especially holds good with regard to 

 the descriptions and drawings by Weché and Marks & Weché (1902 and 1903). 

 Even in this period very valuable monographs on different families of Rotifera ap- 

 pear, especially Rousselet's on Syncbœtadœ (1902), Dixon-Nuttall's on Diaschiza 

 (1903), Jennings on Rattulidœ (1903), Hlava on Melicertidcc (1908 a). Whereas these 

 monographs augment our knowledge very much with regard to the females, the 

 males are either not mentioned at all (Rattulida-) or only rather cursorily mentioned 

 and figured. Apart from a verj' superficial drawing and description oi Aniircpa aciileata 

 by Montet (1915) after 1908, as far as I know, no new males are described. 



If now we will try to collect in a few lines our present knowledge with regard 

 to the males of Rotifers it may be expressed as follows. Of the more than 1000 

 Rotifers described the males have only been observed in a little more than 100 

 species, but of these scarcely one score may be said to have been more exhaustively 

 studied. Of the 25 families of Rotifera the males are wholly unknown in eight 

 (Philodinida', Adinetadœ, Microcodidce, Rattiilidæ, Gastropodida', Ploesomatida', Calhijpnadœ 

 and Anapodidce). In several of the others which contain a large number of species, 

 only the male of a single one or a few has been observed and often only once. 

 The male of the most common Rotifer A. cochlearis is unknown. This in other words 

 means, that our whole present knowledge of the Rotifera has hithertho been built almost 

 exclusively upon the female sex; further that all systematical arrangement of the group 

 has only been tried with regard to this very sex. This rather peculiar result of so many 

 exhaustive and elaborate studies, carried on for about two hundred years and most pro- 

 bably almost unique in the history of Zoology, is due to a series of very intelligible factors. 



The males are generally regarded as verj' rare, much rarer than the females; this 

 may really be true for several genera and perhaps families, but according to mj' ex- 

 perience only for relatively few ; the fact is that the males appear only in strongly marked 

 periods in the lifetime of the species; these periods are bound to fairly fixed seasons 

 of the year, the so-called sexual periods, which for many species occur twice a year, 

 in spring and autumn, for others only once a year, and then almost always at the 

 highest summer temperatures; in a few cases during the winter season, and then 

 even at temperatures near zero, below the ice. The first-named species are termed poly- 

 cyclic, the others monocyclic. Almost simultaneously Lauterborn and myself have 

 tried to elucidate the periodicity of the Rotifera; Lauterborn's papers appeared in 



