34 LIFE ON THE EARTH. 



expanded organ, which can again be drawn through 

 the water more slowly and with less surface to be 

 again expanded and presented for a fresh effort. 



Thus whales and fishes, the Triton and the 

 young Frog, all work the expanded tail after the 

 manner which men use in sculling a boat ; while the 

 beautiful boat-beetle (Notonecta) floating on his back 

 rows himself with long jointed, flattened oars, fringed 

 with stiff bristles. When he strikes for motion, the 

 oars extend themselves, and the bristles catch the 

 water and widen the instrument; but in returning 

 the oar is bent, and turned edgeways, and the bristles 

 pass easily through the liquid Oav^a l$eo-0ai a 

 wonderful thing to behold. 



The paddles of the Turtle and the pectoral fins of 

 the Whale are instruments having the same general 

 property of varying the extent of surface exposed to 

 the water. A similar result is obtained by the very 

 different contrivance of the webbed feet of the Swan 

 and the Otter and that remarkable organ for back- 

 ward swimming or rather leaping in the water, the 

 bending tail of the Lobster and Crawfish, which 

 besides has its five plates endowed with lateral 

 motion, so as to fold up into a small area. 



It is interesting to observe this same structure 

 in the Lobsters (Glyphia) of the ancient oolites, and 

 to note in the Speeton clay and the London clay the 



