236 AN INTRODUCTION TO THE STUDY OF FOSSILS 



by a common axis like the leaves of a book. A fresh supply of 

 water is forced through the siphon into the pallial cavity, and 

 hence the gill is constantly bathed in water abundantly supplied 

 with oxygen. (The structure of the gills and the digestive 

 organs is similar to that in Venus, Fig. 90.) 



The impure blood from the body is brought to the gill by a 

 large vein and is forced into these gill tubes, through the thin wall 

 of which the carbon dioxid carried by the blood is exchanged for 

 the oxygen carried by the water. It is probable that the 

 function of respiration is likewise in part performed by that 

 portion of the mantle surface which is exposed to the water, as 

 in most mollusks this portion is crowded with thin-walled 

 capillaries. 



The digestive system begins with the mouth situated at the 

 anterior end of the alimentary canal. Usually, however, the 

 mouth is not visible, being pulled back into the oesophagus. 

 The peculiar mechanism which accomplishes this withdrawal of 

 the mouth is called an introvert ; it works like the pulling in of 

 the finger of a glove by a thread from within. During its in- 

 vagination the wall of the digestive canal is folded twice on itself, 

 thus pulling inward its anterior extremity, the mouth. During 

 evagination this fold partially straightens out and the mouth is 

 thus pushed out to occupy an anterior position. 



The mouth leads into the buccal cavity, an enlargement of 

 the digestive canal which in turn opens into the oesophagus. 

 The muscular walls of this cavity bear the apparatus for mastica- 

 tion, two small chitinous jaws or mandibles, and the radula, 

 a thin membranous ribbon bearing transverse rows of many tiny 

 chitinous teeth. The radula is borne on a cartilaginous support, 

 — the odontophore, furnished with protractor and retractor 

 muscles by whose action the radula may be sent out through 

 the mouth and may work to and fro like a rasp upon the animal's 

 prey. The extrusion of the mouth through the agency of the 

 introvert carries with it likewise the buccal cavity with its in- 

 cluded radula, thus placing the animal in position for feeding. 



