48 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 papillz 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. 
The Lateral Line 
The sense organs, generally known &s the /ateral 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- 
