14 CLASS MOI.LUSCA. 



periwinkle, {Turbo vulgaris,) have what is called an 

 operculum, (a lid,) which closes the opening, and 

 protects the inhahitant from injury. In the case of the 

 periwinkle, this lid is of horny nature, but, in many spe- 

 cies it is hard and solid, like the shell itself. 



CLASS MOLLUSCA. 



In noticing the animal of a imivalve shell, the part 

 which more readily at-tracts the attention is the mantle, 

 which covers the head of the creature, something like a 

 hood ; it varies much in form and size in different ge- 

 nera. The eyes, which in the sepia are amazingly large 

 and brilliant, are very minute inmost of the other tribes, 

 although they are frequently visible, and would appear, 

 from their formation, to be of little use as organs of 

 sight; indeed, it is supposed, that in the snail they are 

 devoted to the sense of smelling. 



The organs of motion in the Mollusca, according to 

 their different form and position, give names to most of 

 the orders ; these 'consist of muscular expansions of the 

 body, by means of which the animal swims or drags 

 itself along the ground. The gills, or breathing appa- 

 ratus, are situated internally, and communicate with the 

 air or water, by means of a small canal opening out- 

 wardly. The mouth is usually concealed from view 

 when the creature is at rest ; in some, this organ is 

 furnished with a hard substance, which supplies the 

 place of teeth, while, in others, it is in the form of a 

 projecting tube. The greatest portion of these creatures 



