MOLLUSCA. 15 



Child. The very quick manner in which it 

 draws in its horns, and shrinks into its shell, if it 

 is touched. 



Teacher. What would you determine with 

 respect to the animal from this circumstance ] 



Child. That it has the sense of feeling. 



Teacher. Yes ; and which part appears most 

 sensitive ? 



Child. The horns. 



Teacher. And do you observe how the little 

 animal feels about, and tries with these projec- 

 tions which you call horns 1 They have from 

 thence been termed Tentacula, from the Latin, 

 tentare, to try or feel. How many tentacula 

 have snails ? 



Child. Four. 



Teacher. The tentacula vary in number ; 

 many mollusca have only two. The sense of 

 feeling resides in the nerves. 



Child. Oh ! then the mollusca must have 

 nerves. 



Teacher. Yes they have nerves. What other 

 organ of sense besides the tentacula do you per- 

 ceive ? 



Child. There are black specks on the horns 

 of the snail which appear like eyes. 



Teacher. These specks are the organs of 

 sight, of which the mollusca have never more 

 than one pair. The sense of seeing, however, 

 is not universally possessed by this class of ani- 

 mals. The organs of hearing and smelling have 

 never been discovered among them, but they are 

 supposed to possess the latter from the readiness 



