MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 335 



some influence on it, for very shortly after the death of the 

 animal the shell loses many of the characters which distinguished 

 it during life. How this is effected is not known, although at 

 one time there was an idea that vessels passed into the shell 

 among the fibres of the muscles. The first change in a dead 

 shell is for the epidermis or periostracon to fall off, and the 

 substance of the shell then tends to lose its lustre and become 

 opaque. This might be supposed to be due to the action of 

 the atmosphere on the unprotected element of the shell, but 

 that this is not the only cause is shown by the fact that on the 

 extensive sand hills at Bundoran, with their myriads of individuals 

 of Helix nejiwralis and H. hortensis, it is rare to find a specimen 

 even with a trace of epidermis, yet the shells themselves are as 

 brilliant and translucent examples as can be found anywhere 

 else, as the specimens before you testify. In this case I believe 

 that the epidermis is destroyed by the action of the winds on 

 the sharp angles of the drifting sands. 



It is evident that the function of the shell is to protect the 

 soft parts of the animal, and without its protection the life of the 

 rnoUusk would be a very short one. It also aids in locomotion. 

 Until a few years ago this statement would have been con- 

 sidered to have summed up the whole case. But, here again, 

 as is so generally the case in nature, making a universal state- 

 ment is only a proof of ignorance, for in the year 1884, the late 

 Professor Moseley made the extraordinary discovery that the 

 shells of some of the C/ntomdie contained not only other organs 

 of sense but also eyes. He first found them in a specimen, 

 preserved in spirit, of Schizochiton i7tcisiis, and afterwards 

 recognised them in most of the other genera, although they are 

 absent in the genus Chiton itself. 



These structures are confined to the tegmentum or ex- 

 posed area of the shells, and are absent from the girdle. The 

 entire substance of the tegmentum is traversed by a series of 

 branching canals, which are occupied in the living animal by 

 corresponding ramifications of soft tissue, accompanied by 



