CHAPTER VIII. 



THE SNAIL AND ITS HORXS STRUCTURE OP THE MOUTH OF 



VARIOUS GASTEROPODS — TYRIAN PURPLE. 



Allusion has been made to the eyes, the mouth, 

 and aerating apparatus of the gasteropoda ; and on 

 these and other points of general interest, a few ob- 

 servations are here necessary. All are acquainted with 

 the common snail, but few are aware of the beauty of 

 its structure. When we look at one of these creatures, 

 quietly creeping along in quest of food, we see four soft 

 horns, as they are called, protruded from its head ; of 

 these, the two upper are the longest, and it is upon the 

 tip of each that a minute but distinct and perfect eye is 

 seated ; the other pair are destitute of eyes, and are 

 only organs of touch of exquisite perception. These 

 four horns are well supplied with nerves from a large 

 ganglion, and are hollow tubes, capable of retraction 

 by very curious means. Each horn resembles the 

 finger of a glove, and is retracted by its being in- 

 verted, as if the tip of the glove-finger were pulled in. 

 To efl'ect this, a muscular slip, given ofl" from the fibres 



