314 126 



digitiform processes which are found in the females of Ploesoma, Gastropus, Rattulus, 

 we find again in the male sex. 



Only rarely do we find organs which do not occur in the female sex on the 

 disc of the males. This is however the case with the genus Triarthra, and with some, 

 perhaps almost all Melicertidœ. The disc here carries a peculiar hill; where it exists 

 it always carries the two red eyes, which in this way are brought beyond the wheel- 

 organ and not, as in the females, concealed on a level with it. 



A very peculiar organ is the so-called retrocerebral organ, which was formerly 

 described as "Kalkbeutel" ("brain mass" Hudsox-Gosse) and which has been studied 

 by very many authors. It is justly due to de Beauchamp, that we now have a more 

 thorough comprehension of this organ which is unquestionably the most enigmatic 

 in the Rotifera. Still, there are very many unsolved questions connected with it, 

 especially its physiological significance is still almost wholly unknown. De Beauchamp 

 has found it in 1.5 of the 27 families; B. supposes that it really has existed in all 

 families and regards it as a regressive organ, which has disappeared in the Meli- 

 certidœ and Seisonacea. It consists of a retrocerebral sac, flanked by a commonly 

 double subcerebral gland ; the organ opens inside the wheel-organ with two openings, 

 often placed upon a pair of protuberances; these openings are however very difficult 

 to observe; the organ shows a peculiar correlation with the eye-spots, the unpaired 

 eye being always in contact with the sac and the brain; in those species in which 

 we find paired eye-spots, these are always placed verj' near the openings for the 

 organ. It may be regarded as a glandular invagination of the ectoderm, situated in 

 the nonciliated part of the apical zone of the wheel-organ. With regard to its signi- 

 ficance the organ may mainly be regarded as a secretory one; it is highly probable 

 that the two parts of the organ have not the same significance, that it is by no 

 means the same in the different families and that its development and structure in 

 the same species may differ from specimen to specimen, and be of different deve- 

 lopment especially at different ages. The organ is best studied in Eiichlanis, where 

 de Beauchamp has shown that the contents of the median sac are transformed 

 into vacuoles, which are expelled through the orifices, after which the sac shriAels 

 up to a much smaller circumference. Whether this emptying of the sac is a process 

 taking place at regular intervals or only under special outer or inner conditions we 

 do not know. In Xotommata the secretor}' activity is by no means so large. In the 

 so-called "bourses a calcaire"' the secreted matter is designated as bacterioides, re- 

 garded as an excretion "un depot dans certaines cellules de produits destinés à être 

 éliminés lentement ou à rester là jusqu'à la mort de l'animal (de Beauchamp 1909, 

 p. 174). In the Euchlanidœ, where we have a secretion, the secreted matter was 

 formerly regarded as a poison. De Beauchamp supposes that it plays a rôle in the 

 action of the cilia, perhaps that of lubricating organ and remarks that it is best 

 developed in creeping Rotifers. In my opinion this is a very essential point. The 

 organ is inherited from the Turbellaria as mentioned by de Beauchamp "la similitude 

 d'une glande subcérébrale avec certaines glandes muqueses à col très allongé et réduit 



