4 6 



SEA T SE ORGANS OF FISHES 



broad longitudinal fins, specialized to bottom-living, become 

 fashioned in an ancestral mould; and it seems not unnatu- 

 ral that they tend to reacquire their latent primitive form 

 at an early period. On the other hand, the fin-fold condi- 

 tion of the shark might be less perfectly shown on account 

 of processes of accelerated development. 



4. THE CHARACTERS OF THE SENSE ORGANS OF FISHES 



It has already been seen that the conditions of aquatic 

 living have caused fishes to evolve adaptive structural char- 

 acters, such as body form, specialized metamerism, organs 

 of progression, and dermal investiture. It is not, accord- 

 ingly, unnatural to expect that, from the same causes, the 

 condition of the sense organs may have been strikingly 

 modified. 



The sense of "feeling" -using the word in its general 

 meaning --has been of especial value in fishes, and tactile 

 organs appear to be independently developed in all fish 

 groups whose living habits demand them. In the form of 

 barbels they thus occur in members of the various divis- 

 ions of bony fishes, as cod (cusk, Ophidinui) (Fig. 55), 

 drum-fish, Pogonias (Fig. 56), or sculpin, Hemitripterus 

 (Fig. 57). Their form may be lobate, thread-like, or villose ; 

 they are often surprisingly similar in size, position, and 

 innervation ; they usually appear on the inferior head 

 surface, most often in the anterior throat region, in the 

 position most exposed to tactile impressions. The thread- 

 like barbels of the catfishes (Fig. 58, p. 171) are arranged 

 in pairs about the margin of the mouth ; the longest lat- 

 eral pair is connected with the marginal bone (maxillary) 

 of the upper jaw and directed at will. In other mud-living 

 forms, sturgeons (Fig. 160), the barbels have arisen on the 

 under side of the shovel-like snout, directly in advance of 



