VII MOLLUSGA—THE ALIMENTARY CANAL 185 



through the oesophageal ring, which here, as in the Dwtocardia, sur- 

 rounds the anterior end of the pharynx. 



Whereas the salivary glands are, as a rule, branched tubes or 

 acinose, they are sometimes {Scalariidce, lanthinidce, Cancellanidce) 

 simply tubular or (Doliidce, Xeiiophoridm, etc.) sac-like. 



The passage of the ducts of the salivary glands through the 

 oesophageal ring in the Moiwtocardia may have come about by the 

 shifting back of the ring along the pharynx from its former position 

 in the Diotocardia, where it encircles the anterior end of the pharynx 

 in front of the apertures of these ducts. The salivary ducts would 

 thus necessarily become surrounded by the ring. 



The ducts in the Moiwtocardia become the longer the further the 

 nerve ring shifts back from the mouth and pharynx. They are very long 

 in animals provided with a protrusible proboscis, where the ring lies 

 far back on the oesophagus, behind the non-evaginable portion of the 

 proboscis. The ducts here run along the whole length of the latter. 

 But in those cases in which the oesophageal ring has shifted back more 

 quickly than the ducts have lengthened, the glands lie in front of the 

 ring. In the event of the subsequent lengthening of their ducts, the 

 glands might stretch back outside the ring. The arrangement of 

 the glands in the Toxoglossa and Rachiglossa would thus be explained ; 

 here the greater part of the glands lies behind the ring, although 

 the ducts are said not to pass through it. 



The acid secretion of the sali\'ary glands of certain Prosoh'anchia 

 (species of Dolium, Cassis, Cassida.ria, Tritonmm, Murex) and Opistho- 

 hrancliia (Pleurobranchus, Fleurobmnchidium) contains 2'18-4'25 percent 

 of free sulphuric aeid. These carnivorous animals are able, by means 

 of their proboscides, to bore into other Molluscs and Echinoderms which 

 are protected by calcareous skeletons. The sulphuric acid in their 

 glands probably serves for transmuting the carbonate of lime into 

 sulphate of lime, which can then easily be worked through by the 

 radula. 



(i) Pulmonata. — Two salivary glands (Fig. 157, 10) are always 

 found, their ducts entering the pharynx to the right and left of the 

 boundary between it and the oesophagus. The glands lie on the 

 oesophagus and the anterior part of the stomach in the shape of long, 

 lobate, jagged leaves. In some cases they are acinose or round and 

 compact. 



(c) Opisthobranehia. — The salivary glands, of which only one pair 

 is almost always found, here vary in size and shape still more than in 

 the Pulmonata. These glands, which enter the pharynx, must not be 

 confounded with other glands which in many Opisthobranehia enter 

 the buccal cavity, and are sometimes more strongly developed than 

 the salivary glands. 



Dentalium has no salivary glands opening into the pharynx, for 

 the glandular "cheek pouches" enter the buccal cavity, and two 

 diverticula which lie further back belong to the oesophagus. 



