18 GENEEAL HISTORY OF 



eye-specks are situated in front of the oesophageal bulb, to wliich 

 the teeth are attached, they are termed frontal eyes ; and when 

 behind this bulb, cervical eyes. They are sometimes disposed in a 

 line, side by side, as in Triopthalmm ; at others aiTangcd triangularly, 

 as in Eosphora ; in Cycloglena they foiTQ a cii'cle, and in Theorus a 

 cluster on each side. 



Ehi'enberg ha\Tng discovered the existence of eyes under the form 

 of red specks in Rotaioria, argued from analogy the visual character 

 of the similarly colonized specks in the Polygastrica. Recent accui'ate 

 observations made on the eye-specks of Rotatoria (see section XXIII), 

 prove that they are distinctly defined, have an investing capsule, and, 

 in the words of M. Yalenciennes, (Sur les embranchments inferieui's 

 des Annehdos, Ann. des Sciences Nat. 1850), have a crystal! i tie lens, 

 and, consequently, the essential attributes of visual organs. On the 

 contrary, owing to the extreme minuteness of the Polygastric Ani- 

 malcules, all appearance of definite outline is wanting in the red 

 specks of Polygastrica, and resemblances of them being foimd in the 

 reproductive germs of Algae, many observers do not admit the visual 

 nature of these red specks. 



Section IV. — Distinction letween Infusoria and other Minute Animals 

 and Plants. — In our present state of knowledge, "with respect to 

 organic bodies, there are many diificiilties in the way of determining 

 on such boundaries as may reduce them to weU defined groups. Even 

 the line of demarcation between animals and plants, which, at first 

 sight, might be supposed to be so veiy broad and distinct, upon a 

 more minute consideration, is not easily settled. Nor is this sm'- 

 prising, for if we turn to inorganic nature, we find the chemist is 

 equally at a loss to separate the two grand classes into which he 

 divides those bodies : namely, — ^metals and non-metallic substances. 

 "While, at starting, they offer no resemblance, yet, by slight gradations, 

 the bodies of each division approach the other where characters are 

 still wanting to distinguish them. As examples, we may take the 

 metal Silicium, which is sometimes regarded as a non-metallic body; 

 while, on the other side. Iodine and Bromine resemble metals. In 

 the organic world, no difiiculty is found in separating the mammals, 

 birds, and fishes, from forest trees and floweiing plants ; but, as we 

 descend in each kingdom, the Unes of demarcation become less strong 



