INFUSOEIAL ANIMALCULES. 17 



as the so-called tail iu the genera Rotifer, actinunis, &c. The secoud 

 variety of processes are longer and stronger than bristles fsetcejy 

 rather resembling the tentacles of Entomostraca. Ehrenbcrg has 

 only instanced such in the genera Triarthra. The spur presents the 

 form of a short reti'actile stjde projecting from the neck. The 

 existence of sucking discs (patella), at the end of the tails or foot- 

 like organs of some Infusoria, as at the extremity of the stalks of 

 vorticelkB, and also on the tails of some Rotatoria, has been observed 

 by Ehi'enberg. 



Sectiox III. — Of the Eye Species, or Visual Organs of Infusoria. — 

 Oui' knowledge of the existence of these organs is whoUy attributable 

 to the invention of the achi'omatic microscope. In F. 0. Miiller's 

 work, which contains drawings of the larger number of the animal- 

 cules, lately figured by Ehrenberg, and several of them made with 

 much exactness, though on a very small scale, there is not one of the 

 Polygasti'ica represented as possessing a visual organ, and but one 

 species of the Rotatoria, in which he considered the existence of it 

 established. By referring to the engravings, however, it will be 

 seen that nearly all the Rotatoria have eye-specks, and that many of 

 the genera of the Polygastrica are also furnished with them. If no 

 other proof than this could be obtained, therefore, of the existence 

 of a nervous system in these animated atoms, this might still be 

 taken as a sufficient evidence of the fact. 



One of the smallest, and appai-ently the simplest of the genera of 

 Infusoria, in which the eye is perceived, is Ilicroglena, in which, as 

 in the greater number of cases, the coloiu' or j)igment is red. 



By taking a glance at the tabular distribution of the genera of 

 each family, as given after the general remarks on each, in Part III., 

 in this work. The reader wiU notice, at once, that nimibers of the 

 genera of the Polygastrica are furnished with one eye-speck ; and, 

 in some cases, which however are more rare, with two. (See 

 Section XY.) 



In the Rotatoria, the number and position of these organs may be 

 regarded as excellent characteristics of the genera. In the gi'eater 

 proportion of these, the animalcules have two, and, in some instances, 

 three eyes ; whilst, in one genus, Theortis, as many as seven or eight 

 have been distinctly recog-nized on each side of the head. "When the 



c 



