MODES OF PROGRESSION. 



71 



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

 mer streams every one must have observed. The cuttle- 

 fish uses its long tentacles as oars (Fig. 47) ; and some 

 star-fishes (Comatula, Euryale), use their arms with great 

 adroitness. Finally, there are some insects, which have 

 their limbs constructed for running on the surface of water, 

 as the water-spiders. (Ranatra, Hydrometra). 



Pig. 47. 



198. A large number of animals have the faculty of 

 moving both in the air and on 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 of the aquatic birds and most of the reptiles. 

 The latter have even received the name Amphibia, on this 

 account. Finally, there are some which both walk, fly 

 and swim, as the ducks and water hens ; but, on the other 

 hand, they do not excel in either mode of progression. 



199. However different may appear to us the movements 

 and offices performed by the limbs, according to the element 

 in which they act, we see that they are none the less the 

 effect of the same mechanism. The contraction of the 

 same set of muscles, causes the leg of the stag to bend for 

 leaping, the wing of the bird to flap in the air, the arm of 

 the mole to excavate the earth, and the fin of the whale 

 to strike the water. 



