ALIMENTARY SYSTEM — ODONTOPIIORAL TEETH. 
•280 
Fig. 520. — Track of Helix aspersa 
formed by feeding on the paste and 
lime shading upon a greenhouse roof, 
Christchurch, Hants, June, 1883. 
modes of using the odoiitopliore when feeding are also grai)hically 
sliown wlieu snails have fed upon the paste and whiting mixture so 
frequently ap])lied as a summer shading 
to greenhouse roofs ; tlie serpentine or 
meandering series of more or less pyri- 
form tignres impressed upon the com- 
pound by the licking action of the 
tongue are seen to differ in their 
arrangement according to the species, 
showing that each kind of snail when 
feeding moves its head from side to side 
in a more or less characteristic manner 
as the animal advances, hut generally 
speaking, the method of using the 
odontopl\ore resembles that of the 
tongue of a cat when licking, this action rasping particles from the 
food to which the radnla is applied. 
The Radular or Basal Membrane, to which the teeth are attached 
and which together with the teeth form the radnla, is also formed 
within the radular sac, by the transverse range of secretory cells 
anterior to the odontoblasts siilitting at the ends into fibres, which 
are placed side by side, the whole being supported upon the paired 
lingual cartilages, wherein are numerous hranched cartilage cells 
imbedded within a clear matrix, and to Avhich some of the muscles 
actuating the radnla may be attached. At the base of the cartilages 
and anterior to the free end of the radnla, there is usually a sac-like 
deitression in the door of the buccal cavity, the signidcance of which 
is not as yet understood. 
The Surface of the Radular Membrane may be divided longi- 
tudinally into dve areas, grouped as the Central, the Admedian and 
the Marginal, one or more of which may be dedcient in special 
groups or genera. 
The Central or Rachidian area is the narrow longitudinal mid- 
region or Rachis, and bears the median teeth ; the Admedian areas 
or Pleune are paired and placed at each side of, and adjoining the 
median one ; they bear the asymmetrical lateral teeth, while the two 
Marginal areas are occupied by the marginal teeth or uncini, the 
particular features of each type of tooth being correlated with its 
position upon the I’adular membrane. 
