THE TEETH OF INVEETEBEATES 



39 



is armed with many transverse rows of recurved, 

 spinous teeth. This is called the odontophore. 

 This organ is controlled and moved by powerful 

 muscles which draw it backward or forward, or 

 even protrude it, as in the snail. The teeth vary 

 greatly in number in the many various species. 

 Thus the nautilus may have but thirteen, and the 

 snail 12,000 to 40,000. As the teeth are worn off 

 or lost, the ribbon-like tongue is uncoiled and new 



Fig. 5. — A, Radula of Winkle; B, Two rows of teeth, enlarged. 



teeth are brought into use. The upper part of 

 the mouth is usually lined with a horny substance, 

 against which the sharp-toothed tongue works 

 with a rasp-like motion (Fig. 5). The teeth vary 

 in form, but are usually composed of a base, a 

 shank or stem, and a cutting edge, the latter 

 simple or variously denticulated. The middle 

 row of teeth is called the rachidian; the lateral 

 rows, the pleural teeth, and when an additional 

 row occurs outside of this it is called the uncini. 



