340 



MARINE INVERTEBRATES 



Three rows of teeth from the radula of Fasciolaria trapezium. 



RADULA 



Hidden among the red stripes of muscular fiber which line 

 the inner surface of the throat will be found a fleshy strip cov- 

 ered upon one side with many transverse rows of chitinous teeth. 



This is the lingual 

 ribbon, or radula. 

 Remove as much 

 of it as possible, 

 and examine it 

 with a hand-glass. 

 The radula is a 

 most important 

 organ, and is a unique possession of Mollusca, except the pelecy- 

 pods, or bivalves. Usually it lies coiled up like a spiral when 

 not in use ; its purpose is to tear and rasp food. 



The radula has been the subject of much study by concholo- 

 gists, and has furnished the basis for an arrangement of the nu- 

 merous sections of gasteropods into somewhat well-defined groups. 

 If the observer has a strong glass he will notice that each trans- 

 verse row of teeth on the ribbon consists of a median or central 

 tooth with several points or " cusps." Fulgur has a median tooth 

 with three cusps ; Buccinum one with six. On each side of the 

 median tooth is a lateral tooth, which in Fulgur will be seen to 

 have six cusps, and in Buccinum five. In many patterns of radulte 

 there is, besides the me- 

 dian and lateral teeth, still 

 another paired row, called 

 the marginals. In the 

 accompanying figures are 

 given some examples which 

 show small sections of lin- 

 gual ribbons. While all 

 the radula3 of mollusks are capable of being grouped by their 

 general characteristics, yet in no two species of mollusks are the 

 radulaB quite identical. 



Portion of the radula of Imbricaria marmorata. 



