326 GENERAL HISTOEY OF THE INFUSOEIA. 



is the case in Bursariaflava, where I could always trace the curvature of the 

 vessel toward the opposite side of the body most distinctly. F. Stein strongly 

 questions the external opening of the contractile vesicle in the Vorticellce. 

 Hence it is clear that the explanation of the contractile vesicles as part of a 

 water-vascular system is unproven. 



" Is it, however, established, on the other hand, that the contractile reser- 

 voirs pour back their contents again into the parenchyma whence they re- 

 ceive it, as Yon Siebold says ? And if this is the case, how does it happen ? 

 Everything indicates most strongly that the contractile vesicles are filled out 

 of the vessels during the diastole. We see how, during this process, the 

 swollen part of the vessels near their embouchure gradually or suddenly re- 

 turns to its smallest diameter as the stellate figure vanishes ; and I have 

 observed a part of a vessel inflated with the fluid, originating at the extreme 

 end of the animalcule, traverse the whole distance up to the contractile vesi- 

 cle diuing a single diastole. This phenomenon may be supposed to show that 

 the absorbed fluid which had inflated the vessel into a globule, flowed during 

 the said period into the contractile reservoir. 



" But if there is a fair presumption that the contractile vesicles are filled 

 out of the vessels, the above observations teach us nothing whatever on the 

 question as to where the fluid flows duiing the systole. 



" I have hitherto only become acquainted Tvith one fact relating to this point. 

 In Bursaria Vorticella we may detect the following fact : as soon as the con- 

 tractile vesicle which lies at the posterior end of the body has contracted, we 

 may observe at the margins of the animalcule, in its usual position of swim- 

 ming, that two long narrow cavities originate, filled with transparent coloiu'- 

 less fluid ; and these stretch from opposite the mouth as far as the region of 

 the contractile vesicle. They both gradually enlarge, and thus approach near 

 to the anal point ; here they meet, lose their often very irregular form, and 

 change into the globular : the remaiaing contents of the body are displaced 

 upwards by this ; and then these globular reservoirs contract until they 

 vanish, without it being perceptible where the fluid has been driven to ; after 

 some time the narrow light streaks reappear, and the process is repeated 

 in the way above described. The afferent canals, therefore, are not filled 

 at the commencement of the systole ; but must this not be so much the 

 more expected if the fluid flowed back in the same path as it came in, the 

 vanishing of the contractile vesicle taking place much more rapidly than 

 its production ? 



" I have never yet found in any Infusorium special canals in which the 

 fluid is seen to flow back into the body during the systole, and which would 

 give the means of a perfect ciixulation." 



Nucleus. Nucleolus. — A most important internal organ remains for 

 description, viz. the nucleus. This name, if not accurate, is convenient to de- 

 signate the structiu'e in question : it took its rise in the hypothesis of the 

 unicellular nature of the Ciliata, and has ever since replaced the name 

 " testis," or male spermatic gland, assigned it by Ehrenberg on the sup- 

 position of its being the male reproductive organ in these presumed herma- 

 phrodite beings. Indeed, when viewed as the centre of reproductive activity, 

 or, in Prof. Owen's phraseology, the seat of the ' spermatic force,' the Berlin 

 naturalist's name for it does not appear so inappropriate ; nevertheless no real 

 homology can be said to exist between the testis of higher animals and this 

 body, which, on the contrary, has several points of analogy, at least, with the 

 nucleus of plant-cells ; nor can a hermaphrodite nature be rightly ascribed to 

 the Ciliata. 



The nucleus is present in all the Ciliata, and is mostly very readily seen. 



