ALIMENTARY SYSTEM — DEYELOPMENT OF ODONTOPIIORE. 257 
often posteriorly extended beyond the buccal bulb. It is filled with 
connective tissue and its distal ventral walls bear the Odontoblasts 
(oSois, tooth ; /SXacTTo^, a bud) or tooth-forming cells, a few cells 
with large nuclei and clear protoplasm, as 
in the Pulmonates, or a greater number of 
smaller cells, only distinguishable from 
ordinary epithelial cells by their greater 
length, but whose arrangement always coin- 
cides with that of the teeth they secrete ; 
these, however, after their formation by the 
odontoblasts, become hardened for use by a 
superficial deposit of “enamel” from epithelial cells above the radula, 
and are then of an amber colour, an appearance which is soon lost and 
the teeth blunted, broken and worn 
away when they come into active 
use ; this wearing away and loss of 
the anterior functional part of the 
radula is compensated for by the 
progressive growth fi’om the radula 
sac of newly-formed teeth and their 
supporting membrane, exactly like a 
finger nail on its bed, in which the 
wearing away of the anterior free 
margin is compensated for by its own 
forward growth. According to Dr. Sterki, Liiwix campestris, which is 
closely allied to, if not actually identical with, our AgrioUmax Icevis, 
forms in this way not less than 800 transverse rows of teeth during 
its ordinary life-term, and this implies at least eight entire changes of 
denticles ; some Helices he affirms produce 2,000 or more transverse 
rows, indicating 16 — 18 total changes of teeth by the animal. 
The Development of the radular teeth in the embryo begins in 
the form of mere chitinous nodules, which, in the succeeding trans- 
verse rows, acquire many long and sharply pointed cusps or processes 
before eventually attaining the adult or permanent form ; this 
transitory phase in their development has been termed the Echinate 
(eyftos, a hedgehog) stage. At first there are also fewer longitudinal 
rows of teeth than are afterwards present, the additional rows being 
added at the outer margins and not intei’polated between the rows 
R 
Fig. 515, — Diagrammaiic longitudinal 
section through the posterior end of the 
radula sac of a Pulmonate, showing 
the teeth and basal membrane in process 
of formation (after Rossler). 
d.m. basal membrane of the radula; 
c. formative cells of the basal membrane ; 
od. odontoblasts or formative cells of the 
radular teeth ; /. radular teeth. 
Fig. 514. — Epithelial cells 
from pharynx of Helix pomatia 
L., highly magnified (after Vogt 
and Vung). 
