VISION. 431 



shape of a lens.* A structure very similar to this 

 was found to exist in the eye of the Marex trilouis, 

 with the addition of a distinct iris, perforated so 

 as to form a pupil ; a part which had also heen 

 observed, together with a crystalline lens of very 

 large size, in the Valuta cymhumi, by De Blain- 

 ville-t Thus the visual organs of these Gastero- 

 poda appear to possess every requisite for distinct 

 vision, properly so called. Experiments are said 

 to have been recently made, both by Leuchs, and 

 by Steifensand,! in which a snail was repeatedly 

 observed to avoid a small object presented near 

 the tentaculum ; thus affording evidence of its 

 possessing this sense. 



The accurate investigation of the anatomy of the 

 eyes of insects presents considerable difficulty, both 

 from the minuteness of their parts and from the 

 complication of their structure ; so that notwith- 

 standing the light which has recently been thrown 

 on this interesting subject by the patient and labo- 

 rious researches of entomologists, great obscurity 

 still prevails with regard to the mode in which these 

 diminutive beings exercise the sense of vision. Four 

 descriptions of visual organs are met with in the 

 class of Articulated animals : the first are the sim- 

 ple eyes, or stemmata, as they are termed, which 

 appear as lucid spots, resembling those we have 



* Muller thus confirms the accuracy of Swammerdam's account 

 of the anatomy of the eye of the snail, which liad been contested by 

 Sir E. Home, (Phil. Trans. 1824, p. 4) and other writers. 



t Principes d'Anatomie Comparee, i, 445. 



t Quoted by Muller; ibid, p. 16. These results also corroborate 

 the testimony of Swammerdam, who states that he had obtained 

 proofs that the snail could see by means of these organs. 



