6 ILLINOIS NATURAL HISTORY SURVEY MANUAL 



Made of a material called chitin^ the radula is yellowish and 

 transparent. In the mouth of the animal, it is partly coiled and 

 saddle-shaped. It we remove the radula trom a preserved snail, 

 spread it on a glass slide and examine it with a microscope of 

 rather high power, we see that its upper side is covered with many 

 rows of little projections called teeth. Each row of teeth running 

 crosswise of the radula has a central tooth which is symmetrical. 

 On either side to right and left of this central tooth are many 

 other teeth, which are not nearly so symmetrical. Two com- 



14 13 1? il 10 



Fig. 2. — Cross section of head of a land snail: 1, mouth; 2, radula; 

 3, cartilage bearing jaw; 4, jaw; 5, upper lip; 6, esophagus; 7, core 

 of radula; 8, new teeth forming; 9, mucus glands; 10, orifice of 

 mucus glands; 11, posterior cartilage muscles; 12, cartilage; 13, 

 lower lip; 14, anterior cartilage muscles. 



plete half rows of the teeth on the radula of a snail are shown in 

 fig. 3. These are, of course, greatly magnified. Some of the 

 teeth, still more highly magnified, are shown in fig. 4. The manner 

 in which the teeth lie upon the radula, with a portion bent over 

 or reflexed, is shown in fig. 5. 



The number of teeth on a radula is astonishingly large. 

 Thus, the radula of Polygyra albolabris^ the woodland snail we 

 are studying, has 91 teeth in a single row and, as there are 120 

 rows, the total number of teeth is 10,920. Carnivorous snails 

 do not have so many teeth, and these are differently shaped, 

 being sharp and daggerlike, as those of the land snail Haplotrema 



