ALIMENTARY SYSTEM — ODONTOPIIORAL MUSCLES. 
•2 .38 
already present ; these also originate in the nodular form, pass through 
the echinate stage and in the succeeding transverse rows gradually 
acciuire the characters of true 
marginals ; the laterals also, 
in the same way, increase 
somewhat correspondingly in 
number hy the gradual 
change of what were the 
marginals of the embryonal 
mollnsks into the laterals of 
more mature animals, the 
character of the teeth in the 
longitudinal series and their 
relative position on the mem- 
hrane hecoming modified 
thereby. These changes are 
j)eculiar to the embryonal 
>tage and due to modifications of the odontoblasts, as the same 
grou]) of cells forms all the teeth in the same longitudinal row, this 
fact elucidating the can.se of the unvarying recurrence of deficient, 
deformed or abnormal teeth the whole length of the radula, which has 
l)een so often observed. 
The IMuscular DEVELurMENT of the buccal mass is exceedingly 
comjilieated and varies in arrangement in the different groups, as 
many as twenty distinct extrinsic and intrinsic muscles having been 
distinguished as concerned in the efficient use of the radula alone. 
Fig. 517. -Dissection showing the Extrinsic buccal muscles of Helix asfiersa Mull, (after Howes). 
h.b. buccal bulb ; b.c. constrictor muscles of buccal bulb ; c.ni. columellar muscle ; ^/.w. depressor 
muscles \ /. foot ; l.)u. levator muscles ; o. ommatophore ; ic. uesophagus ; buccal protractors ; 
p.r. pharyngeal retractor ; r.s. radular sac ; s.d. salivary ducts ; t. anterior tentacle. 
their alternating contractions and relaxations causing the radula to 
travel backwards and forwards and thus rasp against any object to 
which it may he apjdied, the ju'esence in their substance of 
‘.rs /'A 
Fig. 510. — Radula of an embryo of Limax cam- 
/•estris shewing the nodular character of the teeth on 
first development and thechangesundergone in passing 
through the Echinate intermediate stage before attain- 
ing the adult form, highly magnified (after .Slerki). 
The lowermost series is that of an adult. 
The perpendicular row of numerals indicates the 
number of transverse rows that have existed on the 
radula, reckoning from their first development. 
The horizontal numerals give the position of the 
individual teeth in the transverse rows reckoning from 
the central tooth. 
