ALIMENTARY SYSTEM — MANDIBLES. 
249 
Tlie jMandibles or Jaws are solid cuticular chitinoiis tliickeuiiigs, 
with acute or deiitated edges, placed at the anterior part of the 
buccal cavity ; they are almost universally present in the Gastropoda, 
serving chiefly as prehensile organs for the purpose of seizing or biting 
off particles of food, and the modiflcations to which they are subject are 
most readily appreciated if, as suggested by Lang, they are considered 
as originating from a circlet of jaws surrounding the entrance to the 
oral cavity; such as are still possessed by Umbrella, Tylodina and 
other Opisthobranchs, and of which special parts have been retained 
by the various genera, specialization being evidenced by an increasing 
siini)licity and by the reduction in number of the primitively numerous 
separate parts, owing to the atrophy and loss of certain of the 
segments and the more or less complete and differing modes of fusion 
of the parts retained. 
The muscular arrangements for the efficient action of the mandibles 
in the pleurognathoiis species have been studied, in Cydortoma, by 
Garnault, who flnds that, although the mandibular muscles are to 
some extent complicated with those actuating the radula and buccal 
bulb generally, yet there are several 
distinct muscles directly concerned in 
their movements, the principal being 
a powerful transverse muscular baud, 
attached by each end to the external 
sides of, and connecting together the 
larger jaws, which by its contraction 
are brought into close apposition above 
the radula, the succeeding and 
alternating separation or re-opening 
of the jaivs being due to tlie relaxa- 
tion of the transverse convergent 
or approximating muscle and the simultaneous contraction of the 
lateral divaricators. 
Though not an invariable rule, it is usual amongst our species for a 
ilefinite type of jaw to be associated with an odontophore of a certain 
character, thus the oxygnath jaw of HyaUnki is usually correlated 
with aculeate marginal teeth, which imply a carnivorous tendency ; 
while the strongly ribbed odontognathoiis jaw is generally found vflth 
(quadrate and minutely cusped marginals, which are usually held to 
indicate a preference for vegetable foods. 
Fig. 501. — Mandibles of Cyclostojjict 
clegans (Miill.), x 10, showing their 
principal actuating muscles (modified 
after Garnault). 
c.m. approximating or convergent 
muscle; d.Jit. lateral divaricator muscles; 
l.p. lateral protractor muscles. 
