ALIMENTARY SYSTEM — ODONTOPHORAL MUSCLES. 
259 
liaimogiobiii testifying to the increased respiratory activity neces- 
sitated by their vigorous movements. 
The Extrinsic muscles pass from the odontophore, chiefly to the 
outer walls of the prosoma, and actuate the entire organ ; they are 
known as Retractor.s, Protractors, Levators, or Depressors, according to 
the function they perform. The Protractors 
pass forwards and downwards from the 
buccal bulb to the cephalic integument 
and protrude the radula; the Levators are 
above the Protractors and affixed to the 
cephalic wall near the anterior tentacles ; 
the Depressors underlie the Protractors and 
pass obliquely backwards, and, with the 
Levators, assist to determine the special 
licking motion of the protruded radula, 
which is then bent over the supporting- 
cartilage in front of the oral aperture ; the 
Retractor is a powei-ful muscle or series of muscles with a bifid or 
multifid attachment to the buccal bulb, and distally affixed to the 
columella of the shell or to the integument itself in the nude species. 
The Intrinsic muscles also consist chiefly of Retractors and Pro- 
tractors which arise from the walls 
of the huccal bulb and are affixed to 
the posterior and anterior parts re- 
spectively of the radula, chiefly con- 
trolling its lesser movements and 
also spreading out the radula during 
its protrusion, thereby divaricating 
and erecting the individual teeth 
for effective use ; the sides again 
converging during retraction and 
forming a somewhat prehensile 
apparatus. All species, however, 
do not use their odontophores in 
precisely the same way, as, ac- 
cording to Dr. Sterki, Planoj-hes 
move the radula from behind, 
forward ; PJiys(t the sides towards the centre, while in Llmna a the 
radular motion is a combination of the two methods ; the vai’ious 
Fig. 519. — Median longitudinal section 
through the prosoma of an Euthyneure 
(modified after Howes), X 3, showing the 
disposition of the Intrinsic radular muscles, 
and incidentally the arrangement of the jaw 
and the organs of the prosoma generally. 
b.c. buccal cavity; f. foot ; /./. inner lip; 
j. jaw or mandible; Lc. lingual cartilage; 
l.L lower lip; l.p. lingual protractors; 
l.y\ lingual retractors ; o. ommatophore ; 
ce. cesophagus ; o.s. orifice of left salivary 
gland ; p.g, pedal gland ; p.r. pharyngeal 
retractors ; r. radula ; r.s. radula sac ; 
s.d. right salivary duct ; t. anterior tentacle. 
Fig. 518. — Muzzle retractor of 
Cyclostoina elt’gaiis iPsVxxW.')^ X 3 
(after Siniroth). 
r.n?. retractor muscles ; l.in. 
muscles, whose action gives the 
licking motion to the odonto- 
phore. 
