248 
ALIMENTARY SYSTEM — BUCCAL CAVITY. 
I'lie entrance to the buccal cavity is furnislied with certain hard 
cuticular formations, known as the jaws and odonto[)hore, wliich are 
distinctly connected together hy a ])artially chitinous membrane, the 
presence of wliicli can be clearly demonstrated when the caustic 
solution used in the preparatory })rocess is not too strong. 
'Fhe mandibles or jaws consist of one or more variously arranged 
hard chitinous structures, more or less encompassing the oral aperture, 
while the radiila, or odontophore, is a linguiform cartilaginous cushion, 
which occupies the floor 
of the mouth cavity, and 
is beset with numerous 
hard recurved denticles, 
serving conjointly with 
the mandibles for the pre- 
hension and comminution 
of food, thus facilitating 
the effective action of the 
digestive fluids of the 
stomach and also of the 
secretions from the sali- 
vaiy glands, whose ducts open into the buccal cavity. 
The interior of the buccal cavity generally is lined hy cylindrical 
ei>ithelial cells, overlaid hy a thick cuticle, but the roof is covered 
with a thin Imt strong and partially chitinous membrane or palatal 
plate, connected anteriorly with the cutting edge of the monognathous 
jaw, and overlaying a thick stratum of comjilexly arranged muscle 
fibres, having a longitudinal, transverse and dorso-ventral direction, 
which would appear t(j confer ui)on the upper jaw great freedom of 
motion in various directions. 
In the Pelecypoda generally there is practically no iiharyngeal 
siiecialization, on account of their mode of life not necessitating the 
active search for food, which consists of minute particles or organisms, 
not needing mastication, brought in hy the ijihalent current which 
their ciliary apparatus i)crpetually e.xcites ; they do not, therefore, 
possess the accessory and i)rehensile organs which characterize the 
(Jastropoda, as the oral aperture o])ens directly into the msophagus, 
although some (jf the more archaic Pelecy])ods still retain a buccal 
cavity with two small and laterally symmetrical glandular sacs, 
which jjerhaps represent the salivary glands of the Gastro])ods. 
Fig. oOO. — Longitudinal section through the imecal 
hull) of a Streptoncure (after Lankester) .showing the 
disposition of the teeth and jaws and the organs of the 
head generally. 
/».r. buccal cavity; h.c. head cavity; j. lateral mandible 
or jaw ; Lc. lingual cartilage ; w. mouth ; a\ esophagus; 
p.iu. lingual protractors, posteriorly to them are the 
retractors ; r, radula or odontophore ; 7\s. radula sac ; 
sinus behind the radular sheath ; v.vi. ventral retractor 
muscles of the buccal mass. 
