312 124 



always dart off in straight lines or in large circles, but during this motion they do 

 not as a rule rotate round themselves. 



It will further be understood that an organ of locomotion destined to move 

 the organism along a screw line cannot be formed like one which is to move it 

 along a straight line. I suppose that a rotating motion is mainly dependent first upon 

 a strong development of a ciliary wreath, sharply defined from the nude coronal 

 disc, and secondly upon the wheel-organ being placed terminally; if this is not the 

 case, and it is nevertheless to be used as a rotating organ, the oblique position of 

 the wheel-organ must be counterbalanced in some other way. Wheel-organs consisting 

 of a cilia-covered disc with no stronger development of a special wreath of cilia is 

 not an organ well fitted for rotation. 



In the above-named rather cursory remarks with regard to the use of the wheel- 

 organ in the Rotifera I have endeavoured to point out some of the main conditions 

 for the understanding of the differences between the organ in the male and the fe- 

 male sex. 



It must firstly be emphasised that the wheel-organ in the male sex is placed 

 almost terminally, very rarely ventrally, and even more terminally in those species 

 where it is more or less ventral in the females. This is in accordance with the fact 

 that the part of the wheel-organ surrounding the mouth and which is the most 

 ventrally placed part is obliterated in the male sex. This is especially the case with 

 the hitherto known males of Xotommata, Copeus, Diglena and partly also with Hi]- 

 datina. The disc is very often vaulted, it may be flattened, but very rarely funnel 

 shaped. It is very often covered with a coating of cilia, almost of the same length 

 and in many cases the ciliary wreath round the disc is but slighly developed. It is 

 just the wheel-organ which we should expect in animals which are incapable of 

 constant rotating movements during the swimming motion. A terminally placed, almost 

 totally nude, flattened disc encircled by a wreath of cilia we only find in the As- 

 planclma-mcLles and just these males rotate during swimming as well as the females. 

 Where auricles exist in the females, they are commonly not present or more slightly 

 developed in the male sex. In the Synchœtadœ I have not been able to trace them ; 

 and in the many figures of Synclui'ta which Rousselet has given, they are also 

 absent; in Ploesoma Hudsoni they are but faintly developed, and occur only in the 

 male sex in some males of the genera Copeus and Xotommata. 



From the ventral part of the disc no special part is set off as a cilia-covered 

 furrow, almost always developed in the female, and here leading down to the mouth. 

 It is only well developed in the males of Rhinops and partly also in Euchlanis, at 

 all events in E. dilatata. 



As mentioned above the disc in the female very often carries elevations of 

 different kinds and on the tops setæ varying in numbers and size in the different 

 species. Common to all these hair structures is the fact that in the females they 

 are arranged in relation to the mouth; they play a rôle either as sense organs or 

 more directly as organs for capturing the prej'; thej' serve as a means of procuring 



