340 MARINE INVERTEBRATES 
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 radule 
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 radule of mollusks are capable of being grouped by their 
general characteristics, yet in no two species of mollusks are the 
radule quite identical. 
Three rows of teeth from the radula of Fasciolaria trapezium. 
Portion of the radula of Imbricaria marmorata. 
