Byes, bars, and other sense organs of the dogfish. 
55 
therefore evident that the direction of the movement of this fin is dependent upon the 
stimulation of any part of a given side and is not related to particular spots on that side. 
Hence the movement probably subserves a general function like swimming rather than 
a special one like the protection of the surface. 
Not only are these fin movements called forth by the obvious tactile stimulation 
of given areas of skin, but, as Lyon (1900) first pointed out, they can be induced by 
moving certain parts of the body. If the end of the tail of a dogfish is seized symmet- 
rically and turned to a given side, the dorsal and anal fins bend toward that side as though 
a tactile stimulus had been applied to that side in what has been called the third tactile 
region. That this reaction is really dependent upon a mechanical stimulation of the 
skin and not upon the activity of more deeply seated sense organs, is seen from the fact 
that the reaction disappears when the skin of the tail is rendered insensitive by about 
twenty minutes’ treatment with a 2 per cent solution of cocaine. Not only can these 
correlated fin movements be called forth by turning the tail, 'out they can also be induced 
by moving the head. If the head of a dogfish is taken hold of symmetrically and turned 
toward a given side the median fins, particularly the anterior dorsal, turn toward that 
side. Thus the tactile surfaces of the dogfish are most intimately concerned with the 
correlated movements of this animal’s fins and in such a way that they are undoubtedly 
significant factors in the animal’s locomotion. 
CONCLUSIONS. 
The eyes of the smooth dogfish are the only receptive organs for light possessed by this 
animal. The dogfish reacts with sufficient accuracy to the details of its retinal images 
to show that it has moderately sharp vision. When the sharpness of its vision is greatly 
reduced, it becomes simply positively phototropic. 
The ears of the dogfish are organs of hearing and are concerned with equilibrium and 
muscular tonus. The removal of their otoliths interferes with hearing but not with 
their two other functions. 
The lateral-line organs are stimulated by vibrations of low frequency and by 
pressure. They are relatively insignificant as organs for the control of equilibrium. 
The ampullae of Lorenzini are stimulated by pressure and are doubtless closely 
related in origin and function to the lateral-line organs. 
The whole integument of the dogfish is a receptive organ for mechanical stimuli. 
From it arise impulses for the movement of the nictitating membrane, and for a com- 
plicated system of correlated fin movements most of which are concerned with loco- 
motion and equilibrium. 
LIST OF REFERENCES. 
Bethe, A. 
1899. Die Locomotion des Haifisches (Scyllium) und ihre Beziehungen zu den einzelnen Gehirn- 
theilen und zum Labyrinth. Archiv fur die gesammte Physiologie, bd. 76, heft 9-10, 
p. 470-493. 
Bigelow, H. B. 
1904. The sense of hearing in the goldfish Carassius auratus L. American Naturalist, vol. 38. 
no. 448, p. 275-284. 
