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nourishment. In the males these hair structures are in a few cases arranged as in 

 the females. This is especially the case with Rhinops, where in my opinion the wheel- 

 organ of the two sexes is almost identical also in Euchlanis and in Hydatina senta 

 we find a similar though somewhat simplilied arrangement; where the mouth 

 lies in the female, there is a cilia-covered spot, surrounded by hairpaths verj' similar 

 to those in the females. In very many cases, especially in the most rudimentaiy 

 males, the whole wheel-organ consists of some bundles of long bristles standing 

 upon a disc, homogeneously covered with very short cilia and encircled by a wreath 

 which is more or less sharply set off from the coating of the disc. It is a wheel- 

 organ which is quite unable to catch any organism destined for food and which has 

 only locomotory significance. 



No males have hitherto been described with two bands of cilia round the disc. 

 In the males of Pedalion and in the Melicertidw there is not the slightest trace of 

 either a furrow or posterior band of cilia so characteristic of the females; in the 

 males of Pterodina and Triarthra no tine coatings of minute cilia below the corona 

 are observed, and in the males of the Flosculariidœ we find nothing which may be 

 compared with the highly elaborated wheel-organ of the females; no stifl" setæ are 

 observed; all that we find is a simple somewhat vaulted terminal disc, encircled by 

 a single uninterrupted band of cilia equally long. It is of great interest to see how 

 the typical wheel-organ of the Rotifers, a cilia-covered disc encircled by a single 

 band of cilia is improved in verj' different waj's and in accordance with the use of 

 the organ, in the two families, the Melicertidœ and Flosculariidœ, and in the two sexes 

 of both. In both families as the animals are sessile in the female sex the organ is 

 here almost exclusively formed and used for catching food, but in the two families 

 one of them being detritus-eaters, the other, the Flosculariidæ, true beasts of prey 

 catching the single organisms which arrive in the funnel-shaped corona and later 

 on lacerating them with their teeth, the wheel-organs are as differently developed as 

 possible. In the male sex in both families and where they play no rôle for procuring 

 food and are only organs of locomotion, they are improved in very different ways 

 from what is the type for the females. However differently the wheel-organs may 

 be developed in the females of the two families, in the males they are of quite the 

 same structure, formed as simply as possible, a ciliary disc encircled by a single, 

 uninterrupted row of cilia; all those structures which in the wheel-organs of the 

 females play a rôle for procuring food, and in the two families develop in very 

 different ways, are here totally absent. 



We are therefore able to conclude that all those dilTerent structures, which in 

 the female sex play a rôle for catching the prey, are absent. On the other hand as 

 the males must be more active in their movements than the females and be able 

 to follow them, stress is laid upon improvement of the organ as strongly locomotive. 



In many cases we find hairs of unquestionably sensitive significance upon the 

 disc. In very many cases e. g. in the Sijncha'tahv, Brachionidœ, Anuneadæ and others 

 we find quite the same an-angements of these hairs in both sexes; also the fieshy 



