THE SENSORY CANAL SYSTEM OF FISHES. 505 
as has been previously pointed out, until the publication of 
Leydig’s papers most authors either regarded its function as 
an electric one or as a system of canals simply for the secretion 
of mucus. 
Leydig drew attention to the relations of the system with 
the auditory organ, and compared the canals, &c., with the 
ampulle; later, however, he suggested that the system was 
one of some unknown function, which he termed a sixth sense. 
The most probable theory is that advanced by Schulze (69) 
in 1861, viz. that this system of canals, &c., is one for the 
perception of wave vibrations and oscillations ; to which it were 
as well, perhaps, to add Krause’s theory, 1875, that they also 
serve to give notice of chemical or physical changes in the 
water. 
Dercum (21), 1879 ; Emory (24), 1880; and Bodenstein (9), 
1882, have each pointed out relations to the auditory organ. 
Beard (7), 1886, in a brilliant paper, showed that the organs 
of this system have some physiological relationship with the 
gill-clefts, and that the auditory and olfactory organs are but 
specialised sense-organs of the sensory canal system. 
Ayers (2), 1892, in a voluminous and highly speculative 
treatise, has also pointed out the relations of the auditory 
organ to the sense-organs of this canal system; and although 
this author sums up many such weighty questions in Vertebrate 
morphology upon very little evidence and in a somewhat hasty 
manner, he has nevertheless brought forward a number of 
points which greatly tend to strengthen Beard’s theory,—one 
of the most important, perhaps, being his discovery that “in 
Elasmobranchs the structural connection between the ear organs 
and the surface canal organs is for a long time maintained after 
the ear has migrated to its internal home, and in some forms 
may be said with truth to persist during the life of the 
individual” (p. 315). 
A number of authors—LEisig (23), Whitman, Leydig—have 
pointed out the relations of the segmental sense-organs of 
Invertebrates to those of Vertebrates. Some of these I shall 
have occasion to discuss in a later paper. 
