94 MODES OF PROGRESSION. 



anatomical poiit. of view these may represent very differem 

 parts. In the Whales, the anterior extremities aij the tail 

 are transformed into fins. In Fishes, the pectoral lins, which 

 represent the arms, and the ventral fins, which represent the 

 legs, are employed for swimming, but they are not the prin- 

 cipal organs; for it is by the tail, or caudal fin, that pro- 

 gression is principally effected. Hence the progression of 

 the fish is precisely that of a boat under the sole guidance 

 of the scull ing-oar. In the same manner as a succes- 

 sion of strokes alternately right and left propels the boat 

 straight forwards, so the fish advances by striking alternately 

 light and left. To advance obliquely, it has only to strike 

 a little more strongly in the direction opposite to that which 

 he wishes to take. The Whales, on the contrary, swim by 

 striking the water up and down ; and it is the same with a 

 few fishes also, such as the rays aiid the soles. The air- 

 bladder facilitates the rising and sinking of the fish, by ena- 

 bling it to vary the specific weight of the body. 



196. Most land animals swim with more or less ease, by 

 simply employing the ordinary motions of walking or leaping. 

 Those which frequent the water, like the beaver, or which 

 feed on marine animals, as the otter and duck, have webbed 

 feet; that is to say, the fingers are united by a membrane, 

 which, when expanded, acts .as a paddle. 



197. There is also a large number of invertebrate animals 

 in which swimming is the principal or the only mode of 

 progression. Lobsters swim by means of their tail, and, like 

 the Whales, strike the water up and down. Other Crustacea 

 have a pair of legs fashioned like oars ; as the posterior legs 

 in sea-crab-^, for example. Many insects likewise, swim 

 with their leirs, wliicli are abundantly fi-inged with hairs to 

 give them su'-face : as the little water boatmen, (Gyrinu-3, 

 Dytiscus,) whose mazy dances on the summer streams every 

 M\e must have observed, "^he cuttle-fish uses its long ten' 



