370 



MOLLUSCA. 



are secreted by the epithelium of the radula sheath, which may be 

 compared to the bed of a nail, or to the persistent pulp of a tooth ; 



for as the front part of the radula 

 is continually being worn away by 

 use, it is replaced from behind by 

 the new growth in the sheath. 



The odontophore can be pro- 

 truded and retracted as a whole 

 by its muscles, and there is pos- 

 sibly a small power of movement 

 of the ribbon itself on the sub- 

 radular membrane. 



MJi 



Fig. 292. Longitudinal section through the 

 buccal mass of Helix (after Keferstein). 

 mouth ; Mh buccal ca% f ity ; M muscles ; 

 Rd radula ; Kn lingual cartilage ; Oe oeso- 

 phagus ; Kf jaw ; Z sheath of radula. 



The teeth are arranged in trans- 



verse rows on the surface of the 

 radula ; their form, size, and number vary in different species, and 

 afford important systematic characters of species, genera, and families. 



The middle tooth of each row is called the central or rachidian tooth ; the 

 teeth on each side of this are called laterals, while the outermost of all are 

 the marginals or uncini. The laterals and marginals sometimes merge into 

 one another, so that the distinction between them is lost. The varying 

 arrangement in number of these different teeth is expressed by formulae : 

 thus the typical formula of the Toxoglossa is 1.0.0.0.1., which means that one 



FIG. 293. One row of teeth of the radula of A, Hclicina tropica, Rhipidoglossa ; B, Bythinia 

 tentantlata, Taenioglossa ; C, two rows of Fasciolaria iarcntina, Rachiglossa (from Perrier). 



