I 



127 315 



à une traînée de sécrétion comme on en connaît chez les Triclades, les Néoméniens 

 etc. est indéniable." The more the Rotifers passed over to being either freeswimming 

 or sessile animals and, in some of their most aberrant forms, real plancton organisms, 

 the organ lost its significance and was graduallj' reduced. 



The retrocerebral organ in the males has hardly ever been studied. As far as 

 I know, it is well figured only by Dixon-Nuttall (1892—94, p. .'^.3.3, PI. XV), but 

 he regarded the whole organ as the brain. It would ha\e been of the greatest interest 

 if the investigation of the males could have brought anything special to light with 

 regard to this organ, its structure or its significance. This is however not the case, 

 staining methods of living animals not having been used. Owing to the slight size 

 this will always be very difficult but is surely possible for some of the largest males 

 of Hydatina, Eiichlanis and Copeus. It is of interest to see that in the two genera 

 Eiichlanis and Copeus, where the organ is most strongly developed in the female, it 

 is also strongest in the males. It has also been observed in the males of the other 

 Notommatidœ, and when it has not been figured and mentioned in the descriptions 

 of the other males, much stress must not be laid upon that point. I think that more 

 elaborate investigations will establish its existence. In the males of Euclilanis and 

 Copeus I have found it to be of exactly the same structure as mentioned by de 

 Beauchamp for the females. Onlj' I have been unable to see the Bacteroides in the 

 median sac in the last-named genus, but that is perhaps morely accidental. In the 

 figures of Euclilanis I have not ventured to draw the protuberances provided with the 

 opening. On material from Esrom lake in October 1922 I however found verj' 

 conspicuous protuberances, corresponding exactlj' to those figured bj' de Beauchamp. 



Alimentary canal. In its most complete shape the alimentary canal consists 

 of eight parts in the female sex. 



1. Vestibulum, which is onh' a depression in the disc of the wheel-organ. 

 2. The Pharynx or îMastax, one of the most peculiar organs of the Rotifera 3. an 

 oesophagus covered with Cuticula and only regarded as a prolongation of the Mastax. 

 4. an oesophagus covered with cilia. 5. The gastral glands commonly in a number 

 of two. 6. The ventricle formed by a single cell-layer; the cells are often arranged 

 serially and separated from each other by means of muscle fibres; they are almost 

 always covered with cjlia and contain stored nutriment or excreta. 7. The intestine, 

 often indistinctly separated from the ventricle. 8. The rectum, in which the excretory 

 organ debouches; often it acts as vesica. 



Even in the female sex all these eight parts are onlj' rarely developed. The 

 oesophagus is extremely short in the Euchlanidce, the intestine is absent in the 

 Seisonidw; rectum and anus in the Gastropodida-, Asplanchna, and Paraseison. Apart 

 from the different forms of mastax, also the other parts of the alimenlary canal 

 shows the greatest possible variation in the different families. 



As well known, the alimentarj' canal is almost always rudimentarj' in the male 

 sex, does not function, and in many cases it is impossible to show only the slightest 

 trace of it. Like other rudimentary organs, its development even in the same species 



