STEtJCTimE AND PHYSIOLOGY OP THE MOLLUSCA. 33 



cancellated textiire, unlike any other shell, except perliajs 

 some of the cardiacece and chamacece. 



Epidermis. All sholls have an outer coat of animal matter 

 called the "epidermis" {or periostracum), sometimes thin and 

 transparent, at others thick and opaque. It is thick and olive- 

 coloured in all fresh-water shells and in many arctic sea-shells 

 (e. g. cyprina and astarte) ; the colours of the land-shells often 

 depend on it ; sometimes it is silky as in helix sericea, or fringed 

 "with hairs as in trichotropis ; in the whelk and some species of 

 triton and conus it is thick and rough, like coarse cloth, and in 

 some modiolas it is drawn out into long beard-like filaments. 



In the cowry and other molluscs with large mantle lobes the 

 epidermis is more or less covered up by an additional layer of 

 shell deposited externally. 



The epidermis has life, but not sensation, like the human 

 Bca-rf-skin ; and it protects the shell against the influence of the 

 weather and chemical agents ; it soon fades or is destroyed after 

 the death of the animal in situations where, whilst living, it 

 would have undergone no change. In the bivalves it is 

 organically connected with the margin of the mantle. 



It is most developed in shells which frequent damp situations, 

 amongst decaying leaves, and in fresh-water shells. AH fresh 

 waters are more or less saturated with carbonic- acid gas, and 

 in limestone countries hold so much lime in solution as to 

 deposit it in the form of tufa on the mussels and other shells.* 

 But in the absence of lime to neutralise the acid the water acts 

 on the shells, and would dissolve them entirely if it were not 

 for their protecting epidermis. As it is, we can often recognise 

 fresh-water shells by the erosion of those parts where the 

 epidermis was thinnest, namely, the points of the spiral sheUs 

 and the umhones of the bivalves, those being also the parts 

 longest exposed. Specimens of melanopsis and hithynia become 

 truncated again and again in the course of their growth, until 

 the adults are sometimes only half the length they should be, 

 Rud the discoidal planorbis sometimes becomes perforated by 

 the removal of its inner whorls ; in these cases the animal 

 closes the break in its shell with new layers. Some of the 

 unios thicken their umbones enormously, and form a layer of 

 animal matter with each new layer of shell, so that the river 

 action is arrested at a succession of steps. 



* As at Tisburj', in Wiltshire, where remarkable specimens of anodons were 

 obtained by the late Miss Benett. 



c3 



