222 TYE : NOTES ON THE EPIDERMIS OF MOLLUSCA. 



been filled up with carbonate of lime are thrown off layer after 

 layer in the form of a symmetrical (in some cases the grotesque) 

 shell. 



A shell being broken in any part remote from the mouth is 

 repaired with shelly matter secreted by the visceral mantle, but 

 without an epidermis ; but if broken at the margin, it is repaired 

 with its epidermal covering, the collar of the mantle (the front 

 edge) alone having the power to produce epidermis which is 

 formed over the shell deposit in all cases where the mollusc can 

 withdraw itself sufficiently into its shell to bring the front edge 

 of the mantle into play in repairing the fracture. 



The epidermis is formed in advance of (i.e. before) the shell, 

 the latter being added as an inner coating ; this may well be seen 

 in Helix aspersa and in the Unionidce during their periods of 

 growth. The epidermis will be found as a chitinous fringe or 

 collar round the mouth of the gasteropod in advance of any car- 

 bonate of lime. 



This covering is in manycases, to a largeextent indestructible 

 as is shewn by its presence upon fossil shells of the glacial epoch, 

 e.g. Cyprina islandica, Saxicava tiorvegica, &c. 



Its office is to preserve the shell against the action of eroding 

 agents. " It has life but not sensation, like the human scarf 

 skin. " 



If a shell be dissolved in a solution of hydrochloric acid it 

 leaves behind it a chitinous skeleton {Claiisdia rugosa shews it 

 well). The shell is probably connected in some way with its 

 chitinous envelope by minute channels which serve to keep it 

 "alive" although shells may lose it without materially shortening 

 their period of existence. 



It varies much in thickness in different species, in some 

 cases being easily rubbed off", in others destroyed by atmospheric 

 action or water e.g. Helix virgata, Neritina fluviatilis. 



In some species it is present only in the young state, being 

 afterwards lost as in Oliva and Cyproca the shells being polished 

 by folds of the mantle. 



J.C., v., July, 1887. 



