DIGESTIVE ORGANS. 31 



Mobius has seen Venus mercenaria, and Cypraea Europaea 

 bored through the shell by Murex erinaceus, and the soft parts 

 eaten.* 



Mr. C. Spence Bate has proposed the following theory of the 

 means by which mollusks make these perforations. f 



u His observations upon the boring of the Buceinum into the 

 shells of other mollusca attributed their power of perforation to 

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

 lingual^ ribbon having no part in the operation. The animal 

 takes two days to perforate the shell of Mytilus edulis, and per- 

 forms the work without the least motion of its shell, as must be 

 the ease whenever a circular hole is bored by mechanical action. 



The sea-water itself is probably the solvent used in boring bv 

 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 tun- 

 nels and excavating caverns in the rock is evidence of this sol- 

 vent power ; and the same theory will probably account for the 

 absorption of the columella in the Purpuridae as well as other 

 instances of absorption by the animal of portions of its shell." 



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

 oount 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 any solvent as the unassisted agent in 

 making such a perforation ; yet, on examining a shell not en- 

 tirely 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 (esophagus which traverses it is 

 much narrowed, and w r hen 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 Voluta, Dolium and some other prosobranchiates. Kefer- 

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



* Zool, Garten, 871 1866. 

 f Kept. Brit. Assoc., 73, 1849. 



