36 LIFE ON THE EARTH. 



again in some of the ciliated Infusoria like Trichoda 

 anas. But nowhere does it reach so high a degree 

 of perfection in air-breathing animals, as in the water- 

 birds alluded to ; in which if we examine the form of 

 the head, the texture and arrangement of the fea- 

 thers, and the elegant bony apparatus of the chest, 

 admitting of much expansion and much contraction, 

 we see how well, in every part, the whole living ma- 

 chine is fitted for its peculiar purpose, and calculated 

 to follow with success even the fishes which glide so 

 easily through their native element. 



Boring into solid substances, by which, in con- 

 trast with all these cases, some marine tribes exca- 

 vate for themselves a dwelling, which they can never 

 leave, is accomplished by several quite different con- 

 trivances. Digging into sand is effected by some bi- 

 valve Mollusca, as Lutraria and Mya, by the action of 

 the same muscular mass called a foot, which enables 

 the Cockle to spring some distance, and the Unio to 

 cut its way through the slime. Pholades bore into 

 chalk and much harder limestone by turning round 

 their body and its sharply serrated shelly covering 

 on the pivot of the foot ; Teredines in a similar 

 manner destroy the substance of ships, and Limnoria 

 terebrans, a Crustacean, eats its way into the wooden 

 piles of harbours. So in early geological times, per- 

 forating Modiolse and Pholades are traced by their 



