16 



FIRST BOOK OF ZOOLOGY. 



tentacles are four in number, a larger and a smaller pair. 

 The larger tentacles are called the superior tentacles ; the 



FIG. 18. LAND -SNAILS CRAWLING. &, Breathing Orifice ; t, Superior Tentacles ; i t, Infe- 

 rior Tentacles; m. Mouth. 



smaller ones, often appearing as mere tubercles, are called the 

 inferior tentacles. (See Fig. 18, s t and i t.) As the snail 

 crawls, the superior tentacles are seen in constant motion, as 

 if the creature were feeling its way about with them. 



The eyes, instead of being at the base of the tentacles, as 

 in the fresh-water snails, are found at the tip of the superior 

 tentacles. 



In the land snails, with few exceptions, the tentacles can 

 be drawn within the head, and for this reason they are also 

 called retractile tentacles. 



While the snail is crawling, if the pupil touch the end 

 of the tentacle with his finger, or, even if he alarm the snail 

 by a sudden jar, he will see the tentacles quickly withdraw 

 within the head. The pupil will observe that the bulbous 



