100 DIGESTIVE ORGANS. 



a current of sea-water passing through the buccal apparatus, the 

 lingual ribbon having no pai't in the operation. The animal 

 takes two days to perforate the shell of MytiluH edulis, and 

 performs the work without the least motion of its shell, as must 

 be the case whenever a circular hole is bored by mechanical 

 action. The sea-water itself is probably the solvent used in 

 boring by the mollusca, being charged with free carbonic acid ; 

 and is directed by them against the object to be bored through 

 the process of respiration, and ciliary currents. The action of 

 sea-water upon limestone coasts in driving tunnels and excavating 

 caverns in the rock is evidence of this solvent power ; and the 

 same theory will probably account for the absorption of the colu- 

 mella in the Purpui-idae as well as other instances of absorption 

 by the animal of portions of its sliell." 



I think that the above theory, ingenious as it is, will not 

 account for the perfectly round hole, with clean-cut vertical walls 

 made by boring mollusks in the shells of their prey ; indeed it 

 is difficult to imagine smy solvent as the unassisted agent in 

 making such a perforation ; yet, on examining a shell not entirely 

 bored through, the bottom of the hole is perfectly smooth, 

 showing no marks of mechanical rasping. 



The oesophagus, as already stated, opens into the upper pos- 

 terior end of the mouth. In those mollusks furnished with a 

 proboscis, that portion of the oesophagus which traverses it is 

 much narrowed, and when the proboscis is retracted it is bent 

 into a sigmoid or coil. In its entire length it is provided with 

 interior longitudinal folds. Its middle is dilated into a sort of 

 crop in Yoluta, Dolium and some other prosobranchiates. Kefer- 

 stein has found in Triton variegahivi, and in Dolium galea that 

 the oesophagus, just behind the lingual wall (ix, 96), is dilated 

 below into a longitudinal pouch which is filled up with a gela- 

 tinous tough mass, projecting into the interior like a ridge : it 

 consists of a hyaline material, with many spindle-shaped or 

 stellate cells with round nuclei. A similar organ has been 

 detected in species of Murex, Yoluta (xv, 82), Ancillaria, etc. 



The stomach in its simplest form (xiv, 76), is a dilatation of 

 the digestive tract into which the hepatic ducts open. In Murex 

 and Buccinum it is rounded and curved so that the origins of 

 the oesophagus and intestine approximate. Jn many of the 

 species a blind sack has been detected in connection with the 

 stomach. In some there are internal lobes or filaments (as in 

 Mitra episcopalis), and in others actual tooth-like bodies for com- 

 pressing the food (Telescopium). In Bythinia, Strombus and 

 Pteroceras the blind sack has been found to contain a firm body, 

 somewhat like the hyaline rod of mussels ; it extends some dis- 

 tance into the cavity of the stomach. 



The tongue of the Bullidse being nearly unarmed, an organ 



