70 MODES OF PROGRESSION. 



gans designated under the general term,j^?i5, although in an 

 anatomical point of view, these may represent very different 

 parts. In the Whales, it is the anterior extremities and 

 the tail which are transformed into fins. In Fishes, the pec- 

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

 which represent the legs, are employed for swimming, but 

 they are not the principal organs ; for it is by the tail 

 or caudal fin, that progression is principally effected. 

 Hence the progression of the fish is precisely that of a 

 boat under the sole guidance of the sculling-oar. In the 

 same manner as a succession of strokes alternately right 

 and left, propels the boat straight forwards, so the fish 

 advances by striking alternately right and left. If he 

 wishes to advance obliquely, he 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 and 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. 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, 

 by being expanded, acts as a paddle. 



197. There is also a large number of invertebrate ani- 

 mals 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 Lupa, for example. Many insects, 

 likewise, swim with their legs, which are abundantly fringed 

 with hairs to give them surface ; as the little water boat- 



