VISION. 



339 



account given by Professor Muller,*has four eyes, situated on 

 the hinder part of the head, and covered with the epidermis, 

 but containing in their interior a spherule, composed of an 

 opaque white substance, surrounded by a black pigment, and 

 penetrated by an optic nerve, which is continued to the brain. 

 On the other hand, Professor Weber found in the Hirudo 

 medicinalis, or common leech, no less than ten minute eyes, 

 arranged in a semicircle, in front of the head, and project- 

 ing a little from the surface of the integument: they present 

 externally a convex, and perfectly transparent cornea; while 

 internally, they are prolonged into cylindrical tubes, con- 

 taining a black pigment;! structures, apparently subservient 

 to a species of vision of a higher order than that which con- 

 sists in the simple recognition of the presence of light. 



No organs having the most distant relation to the sense of 

 vision, have ever been observed in any of the Acephalous, 

 or bivalve Mollusca; but various species of Gasteropoda have 



418 



organs which appear to exercise this sense, situated sometimes 

 at the base, sometimes at the middle, and frequently at the ex- 

 tremity of the tentacula. Of the latter we have examples in 

 the common slug and snail, where these tentacula, or horns, 

 are four in number, and are capable of being protruded and 

 again retracted, by folding inwards like the finger of a glove, 

 at the pleasure of the animal. According to Muller, the eye 

 of the Helix pomatia, represented at E, (Fig. 418;) is situ- 

 ated a little to one side of the rounded extremity, or papilla 

 (p,) of the tentaculum, and is attached to an oval bulb of a 



* Annales des Sciences Naturelles, xxii. 23. 



\ Meckel, Archiv. fur Anatomic und Physiologic; 1824, p. 301. 



* Annales des Sciences Naturelles; xxii. 12. 



