HEARING AND ALLIED SENSES IN FISHES. 
59 
The lateral line in Fundulus heteroclitus . — The lateral-line system in F. heteroclitus 
presents a condition typical for teleosts. Its sense organs are contained in canals 
that open by pores on the surface of the skin. A lateral-line canal as indicated by 
its pores (pi. 9, fig. 5) extends along the side of the trunk from near the tail forward 
to the head. Here the arrangement of the pores (figs. 4, 5) gives evidence of a man- 
dibular, a suborbital, a supraorbital, and an occipital branch. By cutting the fifth 
and the seventh nerves behind the eye (fig. 4), and the lateral-line nerve near the 
pectoral girdle (fig. 5), the innervation of this whole system, except a small tract 
above the gills, can be rendered inoperative; the sense organs in the small tract can 
be easily excised. Fishes that have undergone this operation recover almost inva- 
riably and in a very short time; the integument of their heads is insensitive owing 
to the necessity of cutting the fifth as well as the seventh nerves; but that of 
their trunks, which is of course innervated from spinal sources, retains its normal 
sensitiveness, except so far as the lateral-line organs are concerned. In seeking 
for evidence as to the function of the lateral-line organs, I compared carefully the 
reactions of normal fishes with those in which the nerves of the lateral-line organs 
had been cut. 
When a normal fish is liberated in an aquarium, it swims at once to the bottom. 
Here it may move about excitedly for some minutes, after which it usually begins to 
make upward excursions. At first it will swim only part way to the top, returning 
each time quickly to the bottom. Eventually it may make several quick excursions 
to the upper surface of the Avater, and ultimately may remain there playing about 
close to the top. If now any disturbance is made the fish will again swim at once 
to the bottom, and only after some time Avill it return to the top, in the same cautious 
way as before. Almost any disturbance seems to drive the fish to the bottom — a 
flash of light on the water, a quick but noiseless movement of the observer, or an 
unseen blow on the aquarium, conditions all of which suggest that the movements 
of the fish are of a protective nature. 
To one form of disturbance the fishes were particularly sensitive, and this was 
the slight movement of the whole aquarium that occurred whenever the bass-viol 
string was plucked. This movement could lie produced without the accompanying- 
sound by giving a slight vibratory motion to the beam attached to the sounding- 
board on the aquarium (fig. 2). It Avas remarkable how accurately the fishes 
responded to this stimulus. If the fish Avas playing at the top of the Avater, the 
slightest movement of the aquarium as a Avhole would cause it to descend immedi- 
ately to the bottom; if it Avas on its upward course, it could be checked and made to 
descend at any point; and if it was near the bottom, it could be kept there as long 
as the movement continued. In all of the several hundred trials of this kind that 
I made, I never found a normal fish that would remain high in the water or swim 
upward while such movements were being imparted to the aquarium. Whenever 
the fish was above the bottom, the response was an instantaneous downward course. 
With fishes in which the nerves to the lateral-line organs had been cut, the 
reactions Avere totally different. Such fishes, when left to themselves in an aqua- 
rium, were scarcely distinguishable from normal ones. As with the toad-fishes 
observed by Lee (1898, p. 140), the loss of the lateral-line organs seemed to interfere 
in no essential respects with the movements of the animals; they were active and 
quick, returned at once to a normal position when displaced, and oriented with 
