M. N. Lieberkuhn un the Anatomy oj the Infusoria. 321 



once ascertained by compressing the isolated oral structure while 

 the membrane still vibrates. 



Close by the oral slit, on its concave side, lies the pigment-spot. 

 Its form is extremely irregular, sometimes globular, sometimes 

 ellipsoidal, in many cases toothed. Ordinarily it is so distinct 

 as to be at once perceived; sometimes, however, it is so small 

 that it can only be detected by close examination. In animalcules 

 filled with strongly refracting substances alone, it is always 

 difficult to discover it. The pigment-spot of Opkryoglena atra 

 has, on the whole, more uniformity of form and magnitude. If 

 we squeeze down an Ophryoglena flavicans between the covering 

 glass and the slider, we find that the pigment-spot is composed 

 of a heap of minute, scarcely measurable granules, strongly re- 

 fracting light. I never could discover a lens in the pigment. 

 All the specimens examined by me possessed but a single 

 pigment-spot. Beside this lies always a hitherto unobserved- 

 sti'ucture, the form of which is perfectly described when we call 

 it a watch-glass on a small scale. This watch-glass-like organ is 

 transparent and colourless, and shows no trace of fibrous or any 

 other structure. The circular base has a diameter of aboiit 

 yijjth of a millimetre ; its depth amounts to about a third part 

 of this diameter ; the convexity is very considerable. The watch- 

 glass-shaped organ usually turns its convex side towards th(.' 

 pigment-spot ; its concave side is directed towards the point of 

 the head ; it does not seem to be moveable by the animalcule. 

 When isolated, it withstands the action of water for a longer 

 time than is usually the case wath the other parts of the body of 

 this Infusorium. After lying some time in water, it swells up 

 in some degree, and frequently becomes perforated by a hole in 

 the middle. The presence of the watch-glass-shaped organ is 

 not dependent on the presence of a pigment-spot; for Ophryoglena 

 atra possesses a pigment-spot, but no w^atch-glass-shaped organ, 

 while Bursaria jlava has a watch-glass- shaped organ, but no 

 pigment-spot. In other Infusoria with eye-spots, as in the 

 Euglence and Peridinia, I have sought in vain for this organ. I 

 have not met with any facts throwing light on its function. 



The Nucleolus. 



This structure, first described by Von Siebold in Loxodes 

 Bursaria, and subsequently observed by Stein in Prorodon, is 

 properly the only part, except the eye-point, which distinguishes 

 the Ophryoglena in question from Bursaria jlava, — at least in 

 all the specimens which I have hitherto observed. These two 

 animalcules stand, on the whole, much nearer together than 

 Bursaria leucas and Ophryoglena atra, which resemble each 



Ann. c^- Mag. xV. Hist. Ser." 2. Vol xviii. 21 



