MOLLUSCA. 



227 



The germs either enter the body of the oyster in its food, 

 or may be merely attached to the outside and in the 

 liquor of the animals. The danger is greater because 

 oysters are eaten raw or only slightly cooked. 



J\. 





/an.. 



FIG. 74. Helix. A, an empty shell, in section from apex to base, a, apex; an, 

 anus; ap, aperture of shell; c, columella or axis; e, eyestalk; f, foot; /, lip of shell; m, 

 edge of mantle, which secretes the shell; r.o., respiratory opening; s, suture or groove 

 on shell; t, tentacles. B, shows the relation'of the snail to the shell when extended. 



Questions on the Figure. What suggestions of bilateral sym- 

 metry are shown by the snail? Where does growth occur in the 

 shell? What are the functions of the tentacles? Does the snail 

 inhabit the shell clear to the apex? 



247. Definition of the Mollusca. The mollusks agree in having soft 

 unsegmented bodies, a portion of which the mantel usually 

 secretes a protecting shell. The chamber between the mantle and 

 the body contains the respiratory organs. There are no segmented 

 paired appendages, though there is a muscular modification of the 

 body wall which is spoken of as a foot. 



248. Classification of Mollusca. 



No head; gills leaf-like; bivalve shell Class Lamellibranchiata. 



Two equal adductor muscles Order Isomya. 



Examples: Mya (marine clam); Teredo (the ship-worm). 

 Fresh-water mussels; Cardium (cockles). 



