244 FUNCTIONS AND INSTINCTS. 



for themselves a cell in which they may be safe 

 from their enemies ; their food is probably con- 

 veyed to them in the sea water. These animals 

 cannot exist in fresh water, they pierce the wood 

 by means of what Cams calls boring shells 

 moved by a double-bellied muscle. The valves of 

 the shells of this animal are emarginateorbilobed, 

 both lobes are beautifully scored at the margin, 

 but in different directions, the furrows in one 

 being much the finest and receiving those of the 

 other. The mode in which these animals bore 

 has not been ascertained, probably it is by the 

 rotation of their valves. Sir E. Home describes 

 them as protruding a kind of proboscis which 

 has a vermicular motion, and which he supposes 

 to act as a centre-bit while the creature is boring. 

 The shells, by means of their ridges, probably 

 act, like those of the pholads, as rasps. They 

 bore in the direction of the grain of the timber, 

 deviating only to avoid the track of others. 



Various are the animals whose function it is 

 to attack substances from which the vital 

 principle is departed, nor are those, we see in 

 the foregoing instance, which are submerged, 

 always exempted from this law. Fortunately 

 the aquatic animals, that prey upon timber, fall 

 very far short of the terrestrial ones in their 

 number and in the amount of the damage they 

 occasion, and their aversion to fresh water is the 

 safeguard of our bridges and other buildings 



