340' THE SENSORIAL FUNCTIONS. 



black colour. It receives only a slender branch (o) from a 

 large nerve (N N) which is distributed to the papilla of the 

 tentaculum, and appears to be appropriated exclusively to 

 the sense of touch. The bulb, with the eye attached to it, 

 is represented, in this figure, as half retracted within the tu- 

 bular sheath of the tentaculum (s s;) but it can exercise its 

 proper function only when fully exposed, by the complete 

 unfolding and protrusion of the tentaculum. This eye con- 

 tains, within its choroid coat, a semi-fluid and perfectly 

 transparent substance, filling the whole of the globe; and 

 Muller also discovered at the anterior part, another transpa- 

 rent body, having the shape of a lens.* A structure very 

 similar to this was found to exist in the eye of the Murex 

 tritoniSj with the addition of a distinct iris, perforated so as 

 to form a pupil; a part which had also been observed, to- 

 gether with a crystalline lens of very large size, in the Vo- 

 luta cymbium, by De Blainville.t Thus, the visual organs 

 of these Gasteropoda appear to possess every requisite for dis- 

 tinct vision, properly so called. Experiments are said to 

 have been recently made^ both by Leuchs, and by Steifen- 

 sand,J 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 mi- 

 nuteness of their parts and from the complication of their 

 structure; so that notwithstanding the light which has re- 

 cently been thrown on this interesting subject by the patient 

 and laborious researches of entomologists, great obscurity 

 still prevails with regard to the mode in which these dimi- 



* Muller thus confirms the accuracy of Swammerdam's account of the 

 anatomy of the eye of the snail, which had been contested by Sir E. Home 

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



j- Principes d' Anatomic Comparee, i. 445. 



$ Quoted by Muller; ibid. p. 16. These results also corroborate the testi- 

 mony of Swammerdam, who states that he had obtained proofs that the snail 

 could see by means of these organs. 



