46 
BULLETIN of the bureau of fisheries. 
all three fins, i. e., the two dorsal and the anal, are removed the efficiency in swimming 
is somewhat reduced though not as much so as when the caudal fin alone is removed. 
The removal of all the median fins leaves the fish still capable of forward locomotion 
but only with excessive effort, largely because of the small amount of surface that can 
be opposed to the water. The removal of the paired fins from one or both sides has 
very little effect on the swimming of the fish, though its ability to turn accurately is 
much reduced. The removal of all fins both median and lateral leaves the animal still 
capable of wriggling through the water, though with a somewhat rolling motion. It 
is probable that under normal conditions the lateral fins correct this roll. Of all the fins 
the caudal is the one chiefly concerned with locomotion; the others serve mainly as keel- 
like guides and rudders, though the median fins other than the caudal certainly 
supplement this fin in the movements of swimming. 
THE EYES. 
When a normal dogfish is first put into even a large aquarium, it swims about with 
much awkwardness, colliding with such objects as the dark walls and glass sides of the 
aquarium and avoiding only the more conspicuous bodies, such as light-colored rocks, 
etc. The impression given to the observer is that the dogfish has very poor vision, and 
this opinion is current among many fishermen. After a few hours, however, such a 
dogfish will adjust itself to its new quarters and will swim about with only an occasional 
collision. That this condition is not dependent upon its acquaintance with the currents, 
etc., in the aquarium is shown from the fact that if the dogfish is etherized and its optic 
nerves are cut, it will swim slowly about bumping its nose continually against solid 
objects precisely as a blinded animal might be expected to do. Nor does it ever recover 
in any very marked degree from this state. It therefore seems clear that a normal 
dogfish possesses fair vision and that it is capable of adjusting its responses to the stimuli 
in its retinal fields with such precision that its locomotion is in large part guided by these 
stimuli. The relation of the two eyes in these responses is clearly seen when only one 
optic nerve is cut.. Under this condition the dogfish will still swim much as a normal 
one does, though collisions will occasionally occur on its blinded side. Such a fish never 
moves in circles, as many of the lower animals do, showing that the directive discrimina- 
tion in one retinal field is of more importance in its locomotion than the mutual relation 
of the two retinas. 
Not only does a blinded dogfish fail to recognize the detailed illumination of its 
surroundings, but its remaining sensory apparatus is apparently unstimulated by light. 
If a beam of concentrated sunlight is thrown on any part of the skin of a blinded dog- 
fish, no response is obtained, showing that the integumentary nerves of these fishes, unlike 
those of the young lamprey (Parker, 1905 b ) and many amphibians, are not stimulated 
by light. 
Another feature to be observed in the blinded dogfish as compared with the normal 
one is the region of its swimming. A normal dogfish will swim indiscriminately through 
an aquarium, whereas a blinded one remains usually near the bottom and swims about in 
