98 HOM^S WITHOUT HANDS. 



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 irregular, 

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

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

 always hears 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 tiinnelling 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 

 extremity, the aperture at the other end is scarcely capable of 

 receiving the little finger. The walls of this tube are very thin 

 and membranous, and it is more or less retractile, carrying 

 within it the siphons through which the mollusc respires and 

 takes nourishment. 



The Gaper Shell inhabits sandy and muddy shores, and to an 

 inexperienced eye is quite invisible. The shell itself, together 

 with the actual body of the moUusc, is hidden deeply in the 



