56 STRANGE DWELLINGS. 



generally have a bed of muddy matter between their shells and 

 the walls of their dwelling, the Helix saxicava keeps his tunnel 

 perfectly clean and neat. 



' 5th. When the liquor alluded to as forming a fatty aureole 

 round the tunnel penetrates into pre-existing clefts in the rock, 

 it provokes the growth of a microscopic lichen, which also grows 

 in the tunnels in places after the liquor has evaporated. 



' 6th. The tunnels of the Helix saxicava are always irregulai, 

 bearing no relation to the size or shape of the excavators, 

 whereas, in other excavating molluscs, the shape of the hole 

 always bears some relation to its occupant, and also the excava- 

 tions are alike for all animals of the same species.' 



There is an opinion that the gastric juice secreted in the 

 stomach may be the means through which the tunnelling is con- 

 ducted, and that instead of being employed as food within the 

 body it is poured out upon the stone, so. as to dissolve it, the 

 softened substance being then removed by the foot. The Boring 

 Snails do not congregate together during hibernation, as is the 

 well-known custom of the garden.species, but are always solitary. 

 Sometimes two or even three are found in the same burrow, but 

 then they are always at some distance from each other, and form 

 supplementary tunnels of their own. In my own specimen there 

 is a curious example of this peculiarity, where the snail has con- 

 trived to bore completely through the barrier that separates it 

 from a neighbouring tunnel, and has made a hole as large as the 

 keyhole of an ordinary writing-desk, and nearly of the same 

 shape. 



There are many marine boring molluscs, some of which exca- 

 vate mud, others stone, and others timber. Of the mud-borers 

 I have little to say, few of them possessing points worthy of 

 notice. Perhaps the most noteworthy of these is the common 

 Gaper Shell {Mya arenaria), so called, because one end of the 

 shell gapes widely, in order to permit the passage of a long and 

 stout tube. In a specimen now before me, the tube is between 

 three and four inches in length, and at the base is large enough 

 to admit the thumb. As, however, it gradually tapers to the 



