Development of Rotifer vulgaris. 131 



vessels (" respiratory canals "), which run on both sides in the 

 interior of the body-cavity of Rotifer- into the vicinity of the 

 rotatory organ. These canals have throughout a lumen of 

 equal width and a tolerably strong wall ; upon them are 

 seated at various parts the so-called tremulous organs or 

 cilio-faculre, about the structure of which there is still much 

 dispute. I will here briefly state what I have been able to 

 see in them. 



In Rotifer, in which I counted five such organs upon each 

 side, I have ascertained, by the employment of an excellent 

 homogeneous immersion-lens from the workshop of E. Leitz, 

 in Wetzlar (focus j\ inch), with a moderate eyepiece, that 

 each tremulous organ has the form of a cylindrical beaker, 

 seated by its tapering extremity upon the excretory vessel. 

 The beaker is open above, and the broad cilium inserted at its 

 bottom projects a little beyond the aperture. I know very 

 well that my description is imperfect, and that it differs from 

 the representations given by other authors ; but every one 

 can say with a good conscience only what he believes he has 

 observed. We are here on the limit of microscopic vision, 

 and the probability of error is very great. I see, however, 

 that Metschnikoff gives a similar description of the tremulous 

 organ in Apsilus lentifbrmis, saying with regard to that 

 Rotifer, " On each side there are two funnels opening into 

 the body-cavity. In the base of each funnel is seated a long- 

 lobe vibrating in an outward direction " *. From my obser- 

 vations I must perfectly agree with the Russian naturalist, 

 although so distinguished an observer of the Rotifera as Ley- 

 dig states that in the tremulous organs observed by him the 

 direction of the oscillation is inwards. Even under a power 

 of 1500 diameters (Leitz's homogeneous immersion of ^ inch 

 and ocular no. 3) the condition of things appeared to me not 

 otherwise than as above described. I saw that the oscillation 

 of the cilia was so violent that the beakers surrounding them 

 were kept in constant tremulous movement. 



After these observations 1 determined to bring the tremu- 

 lous organs of a female of Brachionus urceolaris into com- 

 parison. A fine large specimen was soon found, and this I 

 examined under exactly the same power as my Rotiferi. The 

 result obtained, however, was not of such a kind that I could 

 employ it in support of my observations on the tremulous 

 organs of Rotifer vulgaris. I will here merely describe what 

 I observed, without troubling myself as to how it is to be 

 brought into accordance with the previous observations upon 

 Rotifer. I took the greatest trouble to discover a contour 

 * Zeitschr. f. wiss. Zool. xvi. p. 349 (1866). 



