300 Account of Professor Ehrenherg's 



single file but several deep. The animals possessing this form 

 of organ are subdivided into two groups, in one of which the 

 margin of the circle formed by the rotatory organ, though inter- 

 rupted by the mouth, is otherwise entire ; in the other the margin 

 is indented or divided into lobes, the first are named Monotrocha, 

 the second Schizotrocha. In its third form the rotatory organ 

 is double, consisting of two circles of cilia, between which 

 the mouth is placed (Fig. 1.) The animals with this form 

 constitute the group of Zygotrocha, to which the common wheel 

 animal {Rotifer vulgaris) belongs. In its fourth and last form, 

 the rotatory apparatus consists of several small wheel-like organs 

 set near to one another, the group in which it so exists are 

 named Polytrocha. The rotatory apparatus of the Hydatina 

 senta affords a good example of this form of organ, and a good 

 representation of it will be found in the plate accompanying Dr 

 Gairdner's paper. Since his first account of the hydatina, how. 

 ever, Dr Ehrenberg has discovered a thickly set circle of curved 

 cilia surrounding the whole rotatory apparatus, also delicate 

 muscular bands which connect the small rotatory organs with 

 each other. 



Such are the chief varieties in the form of these singular or- 

 gans, but by far the most remarkable circumstance in regard to 

 them, is the appearance which they present when in motion. 

 The single and double rotatory organs when set in motion re- 

 semble a toothed wheel turned rapidly round on its axis, first 

 in one direction, then in the opposite, and currents are at the 

 same time produced in the surrounding water, in a regular and 

 determinate direction. In the polytrocha, the wheel-like mo} 

 tion is not obvious, and the currents are excited in no regular 

 or fixed direction, which circumstance serves to distinguish the 

 animals belonging to this group, in cases where, from their 

 smallness, the form of the rotatory apparatus can with difficulty 

 be determined. . 



It has been often a question what actually takes place during 

 this apparent rotation. Baker* supposed that the organ was 

 really constructed like a wheel, and moved freely round on an , 

 axis which formed its sole connection with the rest of the body ; 



* Of Microscopes and the discoveries made thereby. London, 1785; vol. 

 ii. p. 284. 



