LATERAL LINE ORGANS ^ 



face and becomes a series of interconnecting tubes, which 

 pass along the most exposed ridges of forehead, cheek, 

 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 Chimaera 

 (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 Chimaera (Fig. 

 104, M.C)j 



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 

 evidently an adaptation to their peculiar environment, which causes an early 

 ontogenetic stage to be permanently retained. 



f In Callorhynchus this condition has been largely lost : the outer margins 

 of the sensory groove have sealed over. 

 E 



