EYES, EARS, AND OTHER SENSE ORGANS OF THE DOGFISH. 
51 
Although some of these investigators differ among themselves as to the details of their 
conclusions, they all agree in ascribing a function of equilibration to the ear, and this 
conclusion is abundantly borne out by my own observations. If both eighth nerves of a 
smooth dogfish are cut, the animal becomes profoundly disturbed in equilibrium. It 
usually swims in irregular spirals and will rest on the bottom in any position, dorsal or 
ventral side up. When only one nerve is cut, the disturbance is much less pronounced. 
After such an operation a dogfish will often swim and rest in the usual position and be 
almost indistinguishable from a normal individual. If such animals are made to swim 
rapidly, however, they usually show much unsteadiness and may even lose equilibrium. 
A comparison of dogfishes in which one nerve has been cut with those in which both 
have been severed makes it perfectly evident that the loss of one ear can be largely 
compensated for by the other and that it is only after the loss of both ears that profound 
disturbance of equilibrium can be looked for with certainty. These conditions are so 
uniform and clear that the conclusion is fully justified that the ear of the dogfish is a 
receptive organ from which emanate impulses that influence its locomotor mechanism 
so far as to retain the equilibrium of a body that is naturally in a somewhat unstable 
state. 
A dogfish in which one of the eighth nerves has been cut is slightly weaker after the 
operation than before it, and one in which both eighth nerves have been cut is invariably 
very much weaker than it was previously. These differences are very noticeable in 
handling the fishes, and they are characteristic of operations involving the eighth nerves. 
Where, for instance, the second nerves have been cut, this diminution in muscle tonus 
does not occur. It is, as Ewald (1892) has pointed out, a distinguishing feature of the 
eighth nerve. 
From these various observations and experiments on the ears of the smooth dogfish, 
I conclude that these organs, like the ears of the higher vertebrates, are concerned with 
hearing, equilibrium (Flourens), and muscular tonus (Ewald), and that the otoliths are 
not essential to equilibrium, but are in some way concerned with hearing. 
THE ORGANS OF THE LATERAL LINE. 
As I have elsewhere shown (Parker, 1905 a), the lateral-line organs of the smooth 
dogfish can be stimulated by material vibrations of low frequency. This stimulation 
gives rise to movement of the fins, especially of the caudal fin, and to actual locomotion 
in which the fish swims, where possible, downward into deeper water. Lee (1898) has 
maintained on the basis of the movements of the fins as a result of the direct stimulation 
of the lateral-line nerves that the lateral-line organs are concerned with equilibrium and 
that in this respect they are closely related to the ear. I have repeated Lee’s experi- 
ments so far as possible, but with rather different conclusions. 
Lee states that if the lateral-line nerve is cut near its anterior end and stimulated 
centrally, perfectly coordinated, definite movements of the fins occur. Thus if the left 
lateral-line nerve is stimulated, the dorsal fins and caudal fin move to the right, the 
right pectoral and pelvic fins move downward and the left upward. It is true that if 
