MOLLUSC A. 4y 



ing out for danger, and the lower ones for food ; the 

 tiny black dots on the ends of the upper pair, which, 

 however, never seem to see as much as they feel ; the 

 motions of the mouth scraping up food ; and, finally, 

 the division of the foot into head part and crawling 

 disc, — can all be easily seen. (Fig. 25.) 



The long trail of slime left by the crawling disc, and 

 shining aspect of the skin, covered with mucous, are 

 also characteristic of snails.* 



If the foot is observed while crawling on a piece of 

 glass, or at the surface of the water, it will be seen to 

 roll on in little waves. 



The action is that of an elongated sucker, which is 

 applied closely to the surface, and is held there by the 

 pressure of the excluded air or water, but which is 

 capable of lifting parts or strips of its own substance in 

 rapid succession, and pushing them forward in the 

 form of waves. All of these peculiarities afford a fine 

 opportunity for making comparisons Avith the oysters 

 and clams, etc., the Lamellibranchs, whose structural 

 inferiority to the snails is thus easily pointed out. 



An examination of the mouth should be made, and 

 also of the broad, hard upper jaw. This, if taken out 

 and placed under a magnifier, will show a sharp, 

 serrated edge ; and if snails are watched, especially 

 Lymnea, in a water-tank or aquarium, which has been 

 kept a week or so with a few water-plants in it, they 



* It is this function which makes dry sand, ashes, or dust, or 

 any irritating substance, the deadliest means of destroying snails, 

 since they become at once covered \vith the particles, and ex- 

 crete great quantities of mucous in their efforts to get rid of 

 them, until they die from exhaustion. 



