480 THE SENSORIAL FUNCTIONS. 



the genus Eunice of Cuvier,(iycom,Sav.)which^ 

 from the account given by Professor Muller*, 

 has four eyes, situated on the hinder part of the 

 head, and covered with the epidermis ; but con- 

 taining 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 projecting a little from the sur- 

 face of the integument : they present externally 

 a convex, and perfectly transparent cornea; 

 while internally, they are prolonged into cylin- 

 drical tubes, containing a black pigment t ; 

 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 organs 

 which appear to exercise this sense, situated 

 sometimes at the base, sometimes at the middle, 

 and frequently at the extremity of the tenta- 



* Annales des Sciences Naturelles, xxii, 23. 

 t Meckel, Archiv fiir Anatomie und Physiologic; 1824, 

 p. 301. 



