CHAPTER V. 



HEADLESS MOLLUSKS — THE PINNA ITS CABLE THE CLAM — 



THE OYSTER. 



We now proceed to notice the conchiferous, shell- 

 bearing, acephalous, or headless mollusks. These ani- 

 mals are invariably aquatic, and are inclosed in two 

 valves, forming a shelly habitation, to which they are 

 affixed by means of an adductor muscle, stretched from 

 valve to valve. This muscle by its contraction closes 



Headless Mollusk. a a, adductor muscles ; b, foot ; c c, posterior tubes. 



them, and by its relaxation permits them to open, accord- 

 ing to the action of its elastic hinge. 



