158 THE MODES OF PROGRESSION. 



in which swimming is the principal, or the only mode of pro- 

 gression. Lobsters swim by means of a vertical motion of 

 their tail. Other Crustacea have a pah- of legs fashioned like 

 oars ; as the posterior legs in sea crabs, for example. Many 

 insects, likewise, swim with their legs, which are abundantly 

 fringed with hairs, to give them surface ; as the little water 

 boatmen (Gyrinus, Dytiscus), whose mazy dances on the sum- 

 mer streams every one must have observed. The cuttle-fish 

 uses its long arms as oars, and some star-fishes (Cotnatula, 

 Euryale), use their rays with great adroitness. Finally, there 

 are some insects which have their limbs constructed for run- 

 ning on the surface of water, as the water spiders (Ranatra, 

 Hydrometra) . 



306. A large number of animals have the faculty of mo- 

 ving both in the air and on the land, as is the case with most 

 birds, and a large proportion of insects. Others move with equal 

 facility, and by the same members, on land and in water, as 

 some aquatic birds and most reptiles. The latter have received 

 the name amphibia on this account. There are some which 

 walk, fly, and swim, as ducks and water-hens ; but they do 

 not excel in either mode of progression. 



307. However different the movements of the limbs may 

 appear to us, according to the element in which they are per- 

 formed, we see that they are the effect of the same mechanism. 

 The contraction of the same set of muscles, causes the leg of 

 the stag to bend in leaping, the wing of the bird to flap in 

 flying, the arm of the mole to strike outwards in digging, and 

 the fin of the whale to row in swimming. 



