STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 41 



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, arid in 

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



In the cowry and other shell-fish 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 have undergone no change. In the bivalves it is organ- 

 ically 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 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 shells and the umbones of the 

 bivalves, those being also the parts longest exposed. Specimens 

 of melanopsis and bithinia become truncated again and again in 

 the course of their growth, until the adults are sometimes only 

 half the length they should be, and the discoidal planorbis some- 

 times becomes perforated by the removal of its inner whirls ; 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. 



