LATERAL LINE ORGANS 49 
face and becomes a series of interconnecting tubes, which 
pass along the most exposed ridges of forehead, cheek, 
cy orbit, and jaw rim. Here in different regions, these sen- 
sory mucous tubes may become dilated, constricted, or 
ramose, and may communicate with the surface by occa- 
sional or numerous pores. 
The mucous canal system has long been a subject of 
study and investigation. It is looked upon generally as a 
sensory organ, adapted to the conditions of aquatic living, 
but its function has not been definitely established. How 
it was acquired, or how its ancestral conditions have been 
modified in the present groups of fishes, must at present 
be looked upon as in many ways doubtful. 
The simplest conditions of the mucous canal system 
appear to exist in primitive sharks: and to these the 
writer believes that the modified sense canals in other 
fishes may best be referred. 
The ancestral condition of the lateral line of sharks 
appears to have been represented in an open continuous 
groove,* lined with ciliated sense cells, and protected 
only by an overcropping margin of shagreen denticles 
_ (Fig. 61). In this condition it at least exists in the 
ancient sharks of Figs. 86, 87, 92, and in the Chimera 
(Fig. 104). That the canals of the head region were also 
primitively of this character appears exceedingly prob- 
_ able: they are thus retained in the adult Chimera (Fig. 
— 104, W.C).+ 
_ In the modern forms of sharks the condition of the 
___-* It is to be noted that this condition occurs in deep-sea fishes: it here is 
4 evidently an adaptation to their peculiar environment, which causes an early 
_ ontogenetic stage to be permanently retained. 
___ ¢ In Callorhynchus this condition has been largely lost: the outer margins 
of the sensory groove have sealed over. 
