21 209 



mouth parts of the Brachionidcc) , and where the wheel-organ is finally trans- 

 formed in such a way that a real canal for the food helween the two wreaths 

 is performed, this function is totally lost. In accordance with the fact that the 

 mouthparts are unable to procure food and be forced out into the mouth oix-ning, 

 they are more and more withdrawn from it. A beginning of the transformation 

 already takes place in types with malleoramate mouthparts, but is fully developed 

 in the Philodinidcv and Floscularidæ. They have here, especially in the PhUodinldœ, 

 lost all significance as prehensile organs, and placed in the middle of the ijody, 

 they only play a rôle as triturating masticatory organs. 



In the creeping Rotifera the foot is an organ of locomotion and lixation; in 

 the swimming Rotifera, it is mainly a steering organ; it is reduced or often to- 

 tally lost in most of the real planctonorganisms. As the foot is highly variable 

 and modified in accordance with the main functions it has to fulfil, systematical cha- 

 racters can only in a very slight degree be gathered from this organ. 



The Rotifera possess two pairs of almost conform sensitive organs, which 

 may be designated as the anterior and posterior lateral organs; whereas the posterior 

 preserve their orginal position, this is not the case with the anterior organs, which 

 as a rule meet each other dorsally in the middle line and here coalesce to an un- 

 paired sensitive organ of different form; for this organ the name dorsal organ is 

 often used. Only rarely, f. i. in the Asplanchnadœ and in Apsiliis, they have their 

 primitive position, lying separated laterally before the posterior lateral organs. 



The light preserving organs are one median, commonly larger unpaired eyespot, 

 below the brain, and two paired spots, placed before this, and often in a special part of 

 the ciliary disc. Only rarely all the three eyespots are present, commonly only the 

 unpaired one, more rarely the paired one. With regard to the retrocerebral organ 

 the reader is referred to the excellent studies of de Beauchamp, who was the first to 

 show that this very peculiar organ is typical of verj' many, and most probably 

 of all, the Rotifer-families. It is most strongly developed in the more primitive fa- 

 milies of the Rotifera, and strongly reduced in the typical plancton Rotifera and in 

 the fixed families. 



The nerve system has only been very little studied, thoroughly only in a very 

 few species; it is of interest that everywhere where more thorough investigations 

 have been carried out, these studies have ascertained the presence of suboesophageal 

 as well as pedal ganglia. 



The excretory organs are of almost quite the same structure in the whole 

 group, and present no characters which may be used for the systematical arran- 

 gement of the different families. On the other hand, perhaps just this organ, more 

 than any other, clearly shows the systematical position of the whole group in the 

 animal kingdom. 



With regard to the muscle system it may be pointed out, that in this (juite 

 especially, as well as in the cuticula, and in the arrangement of the lateral or- 

 gans, in the more primitive forms we find certain rather conspicuous indications of 



