PHYLUM MOLLUSCA 



253 



a. A Land-snail 



External Features. - - The body of a snail consists of a head 

 (Fig. i So, Ko.), neck, foot (F), and visceral hump. The head bears 

 two pairs of tentacles (Fii.): (i) a short anterior pair containing 

 the olfactory nerves, and (2) a longer pair containing the eyes. 

 The mouth (I/.) is in front and below the tentacles, and just 

 beneath the mouth is the opening of the pedal mucous gland. 

 The foot is broad and flat (F) ; it is a muscular organ of locomo- 

 tion with a mucous- 

 secreting integu- 

 ment. Both the 

 foot and head may 

 be withdrawn into 

 the shell. 



The spiral shell 

 encloses the visceral 

 hump, consisting of 

 parts of the diges- 

 tive, Circulatory, trance to ma ntle cavity indicated by arrow; D., in- 

 respiratory, excre- testine; F, foot; Fit., tentacles; Ko., head; M., mouth: 



and 



FIG. 180. Diaprram showing the structure of a 

 snail. .4, anus; At, respiratory aperture, the en- 



Mh, mantle cavity; Ml., mantle; R.Ml., free edge of 

 mantle; Sch., shell. (From Schmeil.) 



tory, and repro- 

 ductive systems. 

 The mantle (Fig. 180, 1/7.) lines the shell, and is thin except 

 where it joins the foot; here it forms a thick collar which 

 secretes most of the shell. An opening beneath this collar is 

 the respiratory aperture (At) leading into the mantle cavity. 

 The arms (A) opens just back of this aperture. The genital 

 pore is on the side of the head. 



Anatomy and Physiology. - - DIGESTION. - - The general anat- 

 omy of a snail is shown in Figure 181. The digestive organs 

 include a buccal mass, (esophagus (2), salivary glands (j), crop, 

 stomach (4), digestive glands (5), intestine, rectum (6), and amis (7). 



The food is chiefly, if not entirely, vegetation, such as lettuce. 

 This is scraped up by a horny jaw or mandible and devoured after 



