ii 4 ELEMENTARY BIOLOGY. [CHAP. 



(the adductor muscles), one at each end of the animal, 

 which run from one valve of the shell to the other 

 and prevent their separation. The two valves will 

 now be united only by their ligament. 



2. General form and structure. 



a. In the animal now laid bare may be distin- 

 guished 



a. A dorsal border turned towards the hinge of 

 the shell, and nearly straight. 



(3. A curved ventral border, opposite the dorsal, 



y. A wider anterior end. 



8. A narrower posterior end. 



c. A right and left side. 



b. The mantle or pallium. 



a. A bilobed semitransparent membrane, one lobe 

 lining each valve of the shell. 



/?. The continuity of the two lobes on the dorsal 

 side of the animal; their separation along 

 most of its ventral side, where each forms a 

 thick yellowish free border. 



7. The union of the two pallial lobes, for a short 

 distance, towards the posterior part of their 

 ventral border. 



8. The rudimentary dot sal and "ventral siphons, 

 separated from one another at the point of 

 union y and each marked out by a part of 

 the mantle-edge covered by short hair-like 

 processes : the dorsal siphon completely closed 

 below and forming a narrow oval slit; the 



