138 THE CHAIN OF LH^E. 



form, though in many of the humbler types, Hke that of the 

 famiHar Oyster, the gills are used for the double purpose of 

 aerating the blood and, by their minute vibrating threads or 

 cilia, drifting food to the mouth. 



In the great group of radiated animals, the Protozoa^ Calen- 

 terata^ and Echinodermaia, no air-breathing creature exists, 

 or, in so far as is known, has existed, so that this vast group 

 of animals is limited altogether to the waters ; and this is 

 undoubtedly one mark of its inferiority. 



In the sub-kingdom of the Mollusks the highest class, that 

 of the Cuttle-fishes and Nautili, has been, singularly enough, 

 rejected as unfit for this promotion, though it was early intro- 

 duced, and attains to a high development of muscular energy 

 and nervous power. The group next in order, that of the 

 Snails and their allies, alone ventures in some of its families 

 to assume the 7'dle of air-breathing. As might be expected, in 

 creatures of this stamp the simplest means are employed to effect 

 the result. In the sub-aquatic species the gills are contained in 

 a chamber, where they are protected and kept supplied with 

 water. In the air-breathing species, this gill-chamber is merely 

 emptied of its contents and converted into an air-sac or func- 

 tional lung. Thus a rude and imperfect method of air-breath- 

 ing is contrived, which scarcely separates the animals that possess 

 it from their aquatic relatives, but which nevertheless gives to 

 us the beautiful and varied groups of the Land-snails and of 

 the air-breathing fresh-water Snails. 



In the worms and Crustaceans the gills are placed at the 

 sides of the body, and connected with its several segments. 

 But the Crustaceans, like the Cuttle-fishes, though the highest 

 aquatic type, never become ^.ir-breathers. It is true some of 

 them, like the Land-crabs, liv« in the air ; but they retain their 

 gills, and have to carry with them a supply of water to keep 

 these moist. 



But in order to elevate the Annulose type to the true dignity 

 of air breathing, three new classes had to be introduced, differing 



