_Salivary glands_ are found in most Glossophora. They occur in one or two pairs on each side of the pharynx and oesophagus, the duct usually leading forwards and opening into the anterior part of the pharynx (see Figs. 143, 144). They are exceptionally large in the carnivorous Gasteropoda. In certain genera, _e.g._ _Murex_, _Dolium_, _Cassis_, _Pleurobranchus_, the secretions of these glands are found to contain a considerable proportion (sometimes as much as 4·25 per cent) of free sulphuric acid. This fact was first noticed by Troschel, who, while handling a _Dolium galea_ at Messina, saw the creature spit a jet of saliva upon a marble slab, which immediately produced a brisk effervescence. A number of the genera thus provided bore through the shells of other Mollusca and of Echinoderms, to prey upon their soft tissues, and it is possible that the acid assists in the piercing of the shell by converting the hard carbonate of lime into sulphate of lime, which can easily be removed by the action of the radula.[327] In the majority of the Cephalopoda there are two pairs of salivary glands, one lying on each side of the mouth, the other on the middle of the oesophagus. [Illustration: FIG. 144.--Alimentary canal, etc., of _Sepia officinalis_ L.: _a_, anus; _b.d_, one of the biliary ducts; _b.m_, buccal mass; _c_, coecum; _i_, ink-sac; _i.d_, duct of same; _j_, jaws; _l.l_, lobes of the liver; _oe_, oesophagus; _p_, pancreatic coeca; _r_, rectum; _s.g_, salivary glands; _st_, stomach. (From a specimen in the British Museum.)] [Illustration: FIG. 145.--Gizzard of _Scaphander lignarius_ L.: =A=, showing position with regard to oesophagus (_oe_) and intestine (_i_), the latter being full of comminuted fragments of food; _p_, left plate; _p´_, right plate; _p.ac_, accessory plate; =B=, the plates as seen from the front, with the enveloping membranes removed, lettering as in =A=. Natural size.] [Illustration: FIG. 146.--Section of the stomach of _Melongena_, showing the gastric plates (_g.p_, _g.p_,) for the trituration of food; _b.d_, biliary duct; _g.g_, genital gland; _i_, intestine; _l_, liver; _oe_, oesophagus; _st_, stomach. (After Vanstone.)]