46 SENSE 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, Ophidium) (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 
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