VIII 



SALIVARY GLANDS 



237 



branchiata, 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. 



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 exception- 

 ally large in the carnivorous Gasteropoda. In certain genera, e.g. 

 Murex^ Dolium^ Cassis^ Pleurohranclius^ 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 



7v.c/ 



Fig. 143. — Alimentary canal of Helix aspersa L. : a, anus; b.rl, h.d', right and left 

 biliary ducts ; h.in, buccal mass ; c, crop ; li.rj, hermaphrodite gland; i, intestine ; 

 i.o, opening of same from stomach (pyloric orilice) ; I, l , right and left lobes of 

 liver; m, mouth; oe, oesophagus; r, rectum; s.d, salivary duct; s.ff, salivary 

 gland; st, stomach; t, left tentacle. (After Howes and Marshall, slightly 

 modified.) 



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 

 MoUusca 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.^ 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. 



3. The Oesophagus. — That part of the alimentary canal which 



1 Semon, Biol. Centralbl. ix. p. 80. 



