VISION. 429 



which the light proceeds; this latter purpose re- 

 quiring, as we have seen, a special optical appa- 

 ratus of some degree of complexity. An approach 

 to the formation of a crystalline lens takes place in 

 the genus Eunice of Cuvier, (L?/coris, Sav.) which, 

 from the account given by Professor MuUer,* 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 bluck pigment, 

 and penetrated by an optic nerve, which is con- 

 tinued to the brain. On the other hand. Professor 

 Weber found in the Hitudo medicinalis, or common 

 leech, no less than ten minute eyes, arranged in a 

 semicircle, in front of the head, and projecting a 

 little from the surface of the integument : they 

 present externally a convex, and perfectly transpa- 

 rent cornea ; while internally, they are prolonged 

 into cylindrical tubes, containing a black pig- 

 ment \-\ structures, apparently subservient to a spe- 

 cies of vision of a higher order than that which 

 consists in the simple recognition of the presence 

 of light. 



No organs subservient to vision are in general 

 met with in the Acephalous, or bivalve Mollusca ; 

 Mr. Garner, however, has discovered in the Pecten, 

 Spondylns, and Oysler, small, brilliant, emerald-like 

 eyes, having each a minute nerve, a pupil, a pig- 

 mentum, a striated body, and even a lens, situated 

 at the edge of the mantle where alone such organs 

 could be useful.]: Various species of Gasteropoda 



* Annales ties Sciences Naturelles, xxii. 23. 



t Meckel, Archiv fur Anatoinie unci Physiolo^ie ; 1824, p. 301. 



I Tians. Linn. Soc. xvii, 488. 



