INTERNAL STRUCTURE OF THE MOLLUSCA. 3 



in their horns on being touched, and this is effected 

 by a singular and beautiful apparatus ; the tentacle 

 is lengthened by gradually unfolding itself, and not 

 by being pushed out from the base. Each tentacle 

 is a hollow cylinder, to the apex of which is at- 

 tached a muscle, arising from the retractor muscle 

 of the foot, and by its contraction the tentacle is 

 simply inverted and retracted, like the finger of a 

 tight glove; its protrusion, on the other hand, is 

 effected by the alternate contraction of the circular 

 bands of muscles which compose the walls of the 

 tentacle. As a rule, slugs and snails are more 

 liberally provided with teeth than any other animals 

 in the parish, one of our slugs, for instance, pos- 

 sessing no less than 28,000 ; they are not, how- 

 ever, all in use at the same time. The dental 

 apparatus of our univalves may be described as a 

 tube lined with teeth set upon flattened plates, 

 collectively called the lingual ribbon. One extremity 

 of this ribbon is open and spread out like a tongue, 

 teeth upwards, on the floor of the mouth, so as to 

 occupy, in fact, the same relative position as the 

 tongue in the mammalia ; the roof of the mouth is 

 supplied with a horny plate, against which the open 



B 2 



