25G ANATOMY OF VERTEBRATES. 



tebrata. But what would have been the effect if both arm and 

 fore-arm had also extended freely from the side of the fish, and 

 dangled as a long flexible many-jointed appendage in the water! 

 This higher developement, as it is termed, in relation to the pre- 

 hensile limb of the denizen of dry land, would have been an im- 

 perfection in the structure of the creature which is to cleave the 

 liquid element : in it, therefore, the fore limb is reduced to the 

 smallest proportions consistent with its required functions : the 

 brachial and antibrachial segments are abrogated, or hidden in the 

 trunk : the hand alone projects and can be applied, when the fish 

 darts forward, prone and flat, by flexion of the wrist, to the side 

 of the trunk ; or it may be extended at right angles, with its flat 

 surfaces turned forward and backward, so as to check and arrest 

 more or less suddenly, according to its degree of extension, the 

 progress of the fish ; its breadth may also be diminished or 

 increased by approximating or divaricating the rays. In the act 

 of flexion, the fin slightly rotates and gives an oblique stroke 

 to the water. If one of the pectorals be extended, it will turn 

 the fish in a curve towards that side : if the other only, it 

 will turn it on the opposite side : they thus act as a rudder. For 

 these functions, however, the hand requires as much extra 

 developement in breadth, as reduction in length and thickness ; 

 and this is gained by the addition of ten, twenty, or it may be 

 even a hundred digital rays, beyond the number to which the 

 finders are restricted, in the hand of the higher classes of Verte- 

 brata. We find, moreover, as numerous and striking modifi- 

 cations of the pectoral fins, in adjustment to the peculiar habits 

 of the species in Fishes, as we do of the fore limbs in any of the 

 higher classes. This fin may wield a formidable and special 

 weapon of offence, as in many Siluroid fishes. But the modified 

 hands have a more constant secondary office, that of touch, and 

 are applied to ascertain the nature of surrounding objects, and 

 particularly the character of the bottom of the water in which the 

 fish may live. The tactile action of the pectoral fins may be 

 witnessed when gold fish are transferred to a strange vessel ; they 

 compress their air-bladder, and allow themselves to sink near the 

 bottom, which they sweep as it were, by rapid and delicate vibra- 

 tions of the pectoral fins, apparently ascertaining that no sharp 

 stone or stick projects upwards, which might injure them in their 

 rapid movements round their prison. If the pectorals are to 

 pei'form a special office of exploration, certain digits are liberated 

 from the web, and are specially endowed with nervous power for 

 a finer sense of touch, as we see in the Gurnards, fig. 82 ; in 



