EYES, EARS, AND OTHER SENSE ORGANS OF THE DOGFISH. 47 
such a way as to be almost continually in contact with some solid surface, as though 
relying on its sense of touch for its location. 
If the nictitating membranes of a dogfish are drawn across the eyes and stitched to 
the upper eyelids, the fish does not respond as a blinded fish does, but swims about in 
the most brightly illuminated part of the aquarium. This is usually the top, but it may 
be the bottom if light is admitted from low down on the sides. Such fishes are liable 
to collide with solid bodies in their paths of motion and are doubtless reduced to the 
condition of many lower animals in which the visual organs are not image-forming eyes 
but mere direction eyes, i. e., the fishes are reactive to the presence or absence of light 
and to the direction of a chief source, without, however, being able to respond to the 
details of illumination in their surroundings. This condition is doubtless dependent 
upon the fact that the intercepting nictitating membranes are at best only slightly 
translucent and thus prevent the formation of efficient retinal images. 
When a bright light is brought to the glass side of an aquarium otherwise dark, 
normal dogfishes and those whose eyes are covered with the nictitating membranes will 
gather near it. Very likely a submerged light in clear water could thus be made a lure 
for dogfishes in the night. These reactions, however, cease in a generally illuminated 
field such as surrounds the dogfish during daytime. As might be expected from what 
has already been observed, blinded dogfishes show no response to a single light in an 
otherwise dark field. 
From these observations it is clear that the only part of the dogfish sensitive to light 
is the eye and that the retinal image is an important factor in guiding the locomotion 
of these fishes. In an otherwise unilluminated field dogfishes will swim toward a single 
light, i. e., they are positively phototropic. 
THE EARS. 
The original function attributed to the vertebrate ear was of course that of hearing. 
In 1828 Flourens recorded observations that led to the belief that the ear was also con- 
cerned with equilibrium, and this opinion, though not without its opponents, has been 
supported by Goltz, Mach, Breuer, and others. In 1891 Ewald advanced the view that 
the ear likewise had to do with the maintenance of muscular tonus. These three func- 
tions are the chief ones ascribed to the vertebrate ear. To what extent they are char- 
acteristic of the ears of the dogfish will now be discussed. 
In a previous paper (Parker, 1903), on hearing in fishes', I made the statement, 
recently confirmed by Lafite-Dupont (1907), that the ears, lateral-line organ, and skin 
of the dogfish were not open to stimulation by vibrations such as are produced by a 
bass-viol string and transmitted to this fish through the water. But I also noted that 
this fish was responsive to the same vibrations when it rested on a solid transmitting 
base. It would seem from these observations that the smooth dogfish is at best only 
slightly sensitive to material vibrations, and my subsequent work has shown the correct- 
ness of this opinion. To test the question of hearing in the dogfish, I followed the plan 
previously adopted for Fundulus (Parker, 1903), and experimented in the main with 
three classes of fishes: (1) Normal individuals; (2) those with the eighth nerve cut but 
