The general type of structure is a central tooth, a very few laterals, and an occasional marginal or two; teeth of very uniform size and shape throughout. In the Dibranchiata, marginals are entirely absent, their place being always taken, in the Octopoda, by an accessory plate of varying shape and size. This plate is generally absent in the Decapoda. The central tooth is, in the Octopoda, very strong and characteristic; in Eledone and Octopus it is five-cusped, central cusp strong; in Argonauta unicuspid, in Tremoctopus tricuspid. The laterals are always three in number, the innermost lateral having a tendency to assume the form of the central. In Sepia the two inner laterals are exact reproductions of the central tooth; in Eledone, Sepiola, Loligo, and Sepia, the third lateral is falciform and much the largest.
Fig. 142.—Portion of the radula of Octopus tetracirrhus D. Ch., Naples, × 20.
In Nautilus, the only living representative of the Tetrabranchiata, there are two sickle-shaped marginals on each side, each of which has a small accessory plate at the base. The two laterals and the central tooth are small, very similar to one another, unicuspid on a square base.
Fig. 143.—Alimentary canal of Helix aspersa L.: a, anus; b.d, b.d´, right and left biliary ducts; b.m, buccal mass; c, crop; h.g, hermaphrodite gland; i, intestine; i.o, opening of same from stomach (pyloric orifice); l, l´, right and left lobes of liver; m, mouth; oe, oesophagus; r, rectum; s.d, salivary duct; s.g, salivary gland; st, stomach; t, left tentacle. (After Howes and Marshall, slightly modified.)
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.
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.)
