OYSTERS. 



71 



of these organs, one placed near the anus and the other in front 

 of the mouthf, an arrangement which exists in nearly all other 

 acephala. The first tribe, in which there is but one muscle, is 

 called Monomyaria, and the second Dimyaria. 



6. To the first of these divisions belong the OYSTERS,- 

 Ostrea. The shell of these mollusks is irregular, laminated, and 

 composed of two unequal valves, united by a short ligament 

 lodged at either extremity in a pit, and the hinge is without teeth 

 or projecting lamina. The structure of the animal itself is 





c a 



Fig. 89. ANATOMY OF AN OYSTER. 



very simple. Its mantle, the lobes of which are united above 

 near the hinge (6), has a double fringed border : there is no 

 appearance of foot. The adductor muscle (e) of the valves (com- 

 monly called the heart), is situate near the centre of the body, 

 and the heart (Z), instead of being placed as usual on the back, 



Explanation of Fig. 89. Anatomy of an oyster ; a. one of the valves of 

 the shell ; 6. the hinge ; c. mantle of the left side ; d. portion of tho 

 right lobe of the mantle ; e. the muscle ; f. the mouth ; g. the labial 

 tentacula ; h. the liver ; i. the intestine : j. the anus ; k. the branchiae ; 

 /. the heart. 



6. What are the characters of the family of Ostracea? Where are oysters 

 found? 



