n6 ANIMAL LIFE 



with a capacity only exceeded by the red iron pig- 

 ment which we possess. 



In each of these tribes — those of the worm and of the 

 shrimp and snail — we find many air-breathers. There 

 are earthworms and land leeches, land crabs and 

 wood lice, land snails and land slugs. That these 

 arc descended from aquatic ancestors is shown in 

 their dependence upon moisture. A worm shrivels 

 by heat and is killed by frost. Its delicate netted 

 skin is a gill that can withstand neither extremity of 

 climate to which aerial life exposes it. Hence the 

 tropics with their damp undergrowth, and the deeper 

 soil with its more tenacious moisture, are the chief 

 resorts of these adaptive air-breathers, for it is in 

 such places that perennial damp is secured. 



The breathing of vertebrates. Gill-, lung-, and voice- 

 production. — Fish differ from all animals except the 

 zoophytes in their mode of breathing. By a succession 

 of swallowing movements that suffer little interrup- 

 tion, a fish fills its mouth with water, then closes it 

 tightly, and by compressing its throat and gullet 

 drives the water out through its gill-slits. The 

 margins of the slits are converted into tasselled fringes, 

 and the muscles used in swallowing are so arranged 

 as alternately to open and shut these side exits. The 

 gills of a fish consist of these bundles of red tassels 

 mounted on hoops. Each thread of the tassel contains 

 a V-shaped blood-vessel, one limb of which comes 

 from the heart, whilst the other goes to the body. 

 The red colour is evidence that only a thin skin inter- 



