88 THE GASTROPODA 
wall of the digestive tube is folded twice on itself during invagina- 
tion. This is the pleurembolic proboscis of Ray Lankester (Fig. 72) 
found in the Rachiglossa (Fig. 99, XIII), in certain Toxiglossa, and in 
the Doliidae, Cassididae, and Tritonidae among the proboscidiferous 
Taenioglossa. 7In the Naticidae there is a glandular disc on the 
ventral face of the proboscis which serves to perforate the shells 
of the Lamellibranchs on which they feed, and in the Pneumo- 
dermatidae there are suckers in the same position situated on two 
retractile lobes and either isolated or united to one another. 
The mouth leads into the buccal or pharyngeal cavity, which is 
the first of the principal dilatations of the digestive tract. The 
salivary glands open into it, and the chitinous masticatory sclerites 
are attached to its walls. The whole, together with the muscular 
masses which actuate the masticatory apparatus, forms the buccal 
bulb or pharynx (Fig. 74, A), situated behind the oesophageal nerve- 
collar in the more archaic species, but in front of it in the more 
specialised Gastropoda (Fig. 146). In some carnivorous forms, 
such as Glandina and Testacella, the pharynx may be more or less 
completely evaginated, forming a false pleurecbolic proboscis. The 
chitinous buccal sclerites are of two kinds, mandibular and radular. 
(1) The mandibles are solid cuticular thickenings situated at the 
anterior end of the buccal cavity. In the majority of Streptoneura 
and Opisthobranchs they are paired, the members of the pair being 
lateral and symmetrical (Fig. 73, A); they are smooth or scaly, 
generally with trenchant, but sometimes with denticulated margins. 
These paired mandibles are usually quite separate from one another, 
but in the Naticidae they are in contact dorsally, and in Lamellurta 
they are clearly fused together on the dorsal side to form a single 
piece. Similarly there is only a single median mandible formed by 
the fusion of two symmetrical pieces in the Patellidae, in Aegirus 
(Doridomorpha, Fig. 73, B), and in 
all the Pulmonates. This median 
mandible is dorsal, its lower or free 
border is trenchant, nearly horizontal, 
and frequently provided with a median 
projection. Two lateral and sym- 
metrical accessory cuticular thicken- 
Fic. 73. ings are found in the majority of 
Mandibles of Gastropoda. A, paired the basommatophorous Pulmonates 
lateral mandibles of Jans; 2 doreal (Linynaea, Planorbis, ete.). In cer- 
tain Aplysiomorpha in which the 
mandibles are ventral there is a patch of horny spines on the 
root of the buccal cavity (Notarchus), and in certain cases these are 
divided into two symmetrical groups enclosed in diverticula, which 
have the form of evaginable sacs (Gymnosomata). In the Rachi- 
glossa the mandibles are rudimentary, and they are absent in many 
