4 8 



BARBELS AND LATERAL LINE 



the protractile sucking mouth. There can be little doubt 

 that the most aberrant tactile organ in fishes is the long 

 spatulate rostrum of the paddle-fish (Polyodon) of the Mis- 

 sissippi (Fig. 59) : the sense organs are here known to be 

 most highly specialized, although their intimate structure 

 is as yet not understood. Tactile organs are often to be 

 found upon fin structures, especially those of the anterior 

 body region. In the sea-robin, Prionotus (Fig. 60), the sen- 

 sory structures are borne by three anterior fin rays ; these 

 are greatly enlarged, lose their connecting fin web, and 

 can be moved at will in a variety of ways. In all cases 

 the barbels appear to be true and highly specialized 

 organs of touch, and the end organs are comparable ap- 

 parently with the touch papillae of higher forms. Of their 

 extreme sensitivity there can be no doubt, and as far as 

 can be judged from their innervation, it would appear that 

 their function is tactile rather than gustatory, as has been 

 suggested. The limits of these processes, however, are 

 no doubt poorly defined in aquatic living. 



TJie Lateral Line 



The sense organs, generally known as the lateral line, 

 or mucous canal system, are looked upon as essentially 

 peculiar to fishes. In the form of a ' lateral line,' they 

 are arranged more or less segmentally along the median 

 line of either side of the body and form a conspicuous 

 feature in the outward appearance of the fish (Figs. 87, 

 104, LL, 121, LL, 145, LL). Often by striking colora- 

 tion, the lateral line is rendered even more prominent, 

 passing from the head to the tail as a pale or brightly 

 coloured band, against the dusky side of the fish. In the 

 region of the head, however, this sensory structure is, as 

 a rule, no longer conspicuous : it dips below the skin sur- 



