LIFE ON THE EARTH. 33 



the help of fringes dependent from the edge; in 

 Physalia and Yelella, an air-bladder is raised above 

 the general mass and catches the wind; in Nautilus 

 the complicated air-chambers of the shell with a pipe 

 through them all, appears to be another and more 

 remarkable contrivance, which belonged to the group 

 through all geological time; the air-bladder of fishes 

 has been often celebrated in this respect; and we 

 may add the thick oily integument of the Cetacea 

 which at once balances their heavy bones, maintains 

 the heat of their bodies in the polar oceans, and 

 gives the boat-like form which fits them for motion 

 in water. 



Swimming, though chiefly exhibited by the Arti- 

 culated and Yertebrated divisions of animals, is some- 

 times exercised by the other types. Some of the 

 Infusoria swim by the aid of cilia on the periphery, 

 or about the mouth, others by altering the form of 

 the body. Some of the Cephalopoda employ the 

 hinder expansion of the body as well as the arms and 

 hydraulic funnel, for motion. But it is in the Arti- 

 culata with jointed feet and among the Vertebrata, 

 that swimming by special organs arrives at the most 

 curious and diversified excellence. 



We may remark in regard to nearly all of these 

 special organs that the general idea carried out in 

 them is that of striking the water forcibly with an 



B.L. 



