LOCOMOTION OF FISHES. 257 



which they also serve as limhs to creep along the bottom, when 

 the fish is exjiloring the sand Avitli its mailed mouth. 



Some Gobioids (Periopthalmus) can use their mnsciilar pecto- 

 rals to shuffle along the shore, or hunt for insects in humid places.' 

 Certain Lophioids living on sand-banks that are left dry at low 

 water are enabled to hop after the retreating tide by a special 

 prolongation of the carpal joint of the pectoral fin, tig. 102; which 

 fin in these 'frog-fishes' projects like the limb of a terrestrial 

 quadruped, and presents two distinct segments clear of the trunk. - 



The sharks, whose form of body and strength of tail enable 

 them to swim near the surface of the ocean, are further adapted 

 lor this sphere of activity and compensated for the absence of an 

 air-bladder by the large proportional size and strength of their 

 pectoral fins, figs. 30, 104, which take a greater share in their 

 active and varied evolutions than they can do in ordinary fishes. 



The flat-bodied Hays, equally devoid of an air-bladder, and 

 with a long and slender tail, deprived of its ordinary propelling- 

 powers, grovel at the bottom ; but have a still greater develope- 

 ment of the hands, fig. 64, 12, 12, which surpass in breadth the 

 whole trunk, and react with greater force upon it in raising it 

 from tlie bottom, by virtue of a special modification of the scapular 

 arch, which is directly attached to the dorsal vertebraj. 



Nor is the pectoral member restricted in length where its 

 office, in subserviency to the special exigencies of the fish, 

 demands a devclopement in that direction ; the fingers of the 

 Exoccetus and Dactijlopterus, arc as long, and the web which they 

 sustain as broad, as in the expanded wing of the flying mammal. 

 Everywhere, whatever resemblance or analogy we may perceive 

 in the iehthyie modifications of the Vertebrate skeleton to the 

 lower forms or the embryos of the higher classes, we shall find 

 such analogies to be the result of special adaptations for the \)\\v- 

 pose or function for which that part of the fish is designed. 



The ventral fins or homologues of the hind-legs are still more 

 rudimental — still more embryonic, having in view the comparison 

 with the stages of devclopement in a land animal — than the ] ec- 

 toral fins ; and their small proportional size reminds the homologist 

 of the later appearance of the hind limbs, in the devclopement of 

 the land Vertebrate. But the hind limbs more immediately relate 

 to the support and progression of an animal on dry land than the 

 fore limbs : the legs are the sole terrestrial locomotive organs in 

 Birds, whose fore limbs are exclusively modified, as wings, for 

 motion in another element. The legs are the sole organ of sup- 



' ci.xxiv. viil. iii. p, 97. 

 VOL. I. ^> 



