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. In 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 Lamelkiria 

 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- 

 FIO. 73. ings are found in the majority of 



Mandibles of Gastropoda. A, paired the baSOmmatOphorOUS PulmonatCS 

 lateral mandibles of Janui ; B, dorsal /7"Vw/T/7 Plnnnrhie ofp \ Tn rt*r 

 mandible of AegirM. (After Hancock.) (******, flanOTtoS, CtC.). 



tain Aplysiomorpha in which the 



mandibles are ventral there is a patch of horny spines on the 

 roof 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 



