STRUCTURE AND PHYSIOLOGY OF TI MOLLUSCA. 30 
cancellated texture, unlike any other shell, except perhaps 
some of the cardiacece and chamacece. 
Epidermis. All shells 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 helia sericea, or fringed 
with hairs as in trichotropis ; in the whelk and some species of 
triton and conus 1t 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 
scarf-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 haye 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. All 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 itis, we can often recognise 
fresh-water shells by the erosion of those parts where the 
epidermis was thinnest, namely, the points of the spiral shells 
and the wmbones of the bivalves, those being also the parts 
longest exposed. Specimens of melanopsis and bithynia become 
truncated again and again in the course of their growth, until 
the adults are sometimes only half the length they should be, 
aud 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 Tisbury, in Wiltshire, where remarkable specimens of anodons were 
obtained by the late Miss Benett. 
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