THE ORGANS OF SENSE
=The Organs of Smell.=--The sense-organs of the fish correspond in
general to those of the higher vertebrates. The sense of taste is,
however, feeble or wanting, and that of hearing is muffled and without
power of acute discrimination, if indeed it exists at all. According
to Dr. Kingsley (Vert. Zool., p. 75), "recent experiments tend to show
that in fishes the ears are without auditory functions and are solely
organs of equilibration."
The sense of smell resides in the nostrils, which have no relation
to the work of breathing. No fish breathes through its nostrils, and
only in a few of the lowest forms (hagfishes) does the nostril pierce
through the roof of the mouth. In the bony fishes the nostril is a
single cavity, on either side, lined with delicate or fringed membrane,
well provided with blood-vessels, and with nerves from the olfactory
lobe. In most cases each nasal cavity has two external openings. These
may be simple, or the rim of the nostril may be elevated, forming a
papilla or even a long barbel. Either nostril may have a papilla or
barbel, or the two may unite in one structure with two openings or with
sieve-like openings, or in some degenerate types (_Tropidichthys_)
with no obvious openings at all, the olfactory nerves spreading over
the skin of a small papilla. The openings may be round, slit-like,
pore-like, or may have various other forms. In certain families of bony
fishes (_Pomacentridæ_, _Cichlidæ_, _Hexagrammidæ_), there is but one
opening to each nostril. In the sharks, rays, and chimæras there is
also but one opening on either side and the nostril is large and highly
specialized, with valvular flaps controlled by muscles which are said
to enable them "to scent actively as well as to smell passively."
In the lancelet there is a single median organ supposed to be a
nostril, a small depression at the front of the head, covered by
ciliated membrane. In the hagfish the single median nostril pierces the
roof of the mouth, and is strengthened by cartilaginous rings, like
those of the windpipe. In the lamprey the single median nostril leads
to a blind sac. In the _Barramunda_ (_Neoceratodus_) there are both
external and internal nares, the former being situated just within the
upper lip. In all other fishes there is a nasal sac on either side of
the head. This has usually, but not always, two openings.
There is little doubt that the sense of smell in fishes is relatively
acute, and that the odor of their prey attracts them to it. It is known
that flesh, blood, or a decaying carcass will attract sharks, and other
predatory fish are drawn in a similar manner. At the same time the
strength of this function is yet to be tested by experiments.
[Illustration: FIG. 83.--Dismal Swamp Fish, _Chologaster cornutus_
Agassiz. Supposed ancestor of _Typhlichthys_. Virginia.]
[Illustration: FIG. 84.--Blind Cavefish, _Typhlichthys subterraneus_
Girard. Mammoth Cave, Kentucky.]
=The Organs of Sight.=--The eyes of fishes differ from those of the
higher vertebrates mainly in the spherical form of the crystalline
lens. This extreme convexity is necessary because the lens itself is
not very much denser than the fluid in which the fishes live. The
eyes vary very much in size and somewhat in form and position. They
are larger in fishes living at a moderate depth than in shore fishes
or river fishes. At great depths, as a mile or more, where all light
is lost, they may become aborted or rudimentary, and may be covered
by the skin. Often species with very large eyes, making the most of a
little light or of light from their own luminous spots, will inhabit
the same depths with fishes having very small eyes or eyes apparently
useless for seeing, retained as vestigial structures through heredity.
Fishes which live in caves become also blind, the structures showing
every possible phase of degradation. The details of this gradual loss
of eyes, whether through reversed selection or hypothetically through
inheritance of atrophy produced by disuse, have been given in a number
of memoirs on the blind fishes of the Mississippi Valley by Dr. Carl H.
Eigenmann.
In some fishes the eye is raised on a short, fleshy stalk and can
be moved about at the will of the fish. It is said that the vision
of the pond-skipper, _Periophthalmus_, when hunting insects on the
mud flats of Japan or India is "quite equal to that of a frog." It
is known also that trout possess keen eyesight, and that they show a
marked preference for one sort or another of real or artificial fly.
Nevertheless the vision of fishes in general is probably not very
precise. They apparently notice motion rather than outline, changes
rather than objects, while the extreme curvature of the crystalline
lens would seem to render them all near-sighted.
[Illustration: FIG. 85.--Four-eyed Fish, _Anableps dovii_ Gill.
Tehuantepec, Mexico.]
In the eyes of the fishes there is no lachrymal gland. True eyelids
no fishes possess; the integuments of the head pass over the eye,
becoming transparent as they cross the orbit. In some fishes part of
this integument is thickened, covering the eye fully although still
transparent. This forms the adipose eyelid characteristic of the
mullet, mackerel, and ladyfish. Many of the sharks possess a distinct
nictitating membrane or special eyelid, moved by a set of muscles. The
iris in most fishes surrounds a round pupil without much power of
contraction. It is frequently brightly colored, red, orange, black,
blue, or green. In fishes, like rays or flounders, which lie on the
bottom, a dark lobe covers the upper part of the pupil--a curtain
to shut out light from above. The cornea is little convex, leaving
small space for aqueous humor. In two genera of fishes, _Anableps_,
_Dialommus_, the cornea is divided by a horizontal partition into two
parts. This arrangement permits these fishes, which swim at the surface
of the water, to see both in and out of the medium. _Anableps_, the
four-eyed fish, is a fresh-water fish of tropical America, which swims
at the surface like a top-minnow, feeding on insects. _Dialommus_ is a
marine blenny from the Panama region, apparently of similar habit.
[Illustration: FIG. 86.--_Ipnops murrayi_ Günther.]
In one genus of deep-sea fishes, _Ipnops_, the eyes are spread out to
cover the whole upper surface of the head, being modified as luminous
areas. Whether these fishes can see at all is not known.
[Illustration: FIG. 87.--Pond-skipper, _Boleophthalmus chinensis_
(Osbeck). Bay of Tokyo, Japan; from nature. K. Morita. (Eye-stalks
shrunken in preservation.)]
The position of the optic nerves is described in a previous chapter.
In ordinary fishes there is one eye on each side of the head, but in
the flounders, by a distortion of the cranium, both appear on the same
side. This side is turned uppermost as the fish swims in the water or
when it lies on the bottom. This distortion is a matter of development.
The very young flounder swims with its broad axis vertical in the
water, and it has one eye on either side. As soon as it rests on the
bottom it begins to lean to one side. The lower eye changes its axis
and by degrees travels across the face of the fish, part of the bony
interorbital moving with it across to the other side. In some soles it
is said to pass through the substance of the head, reappearing on the
other side. In all species which the writer has examined the cranium
is twisted, the eye moving with the bones; and the frontal bone is
divided, a new orbit being formed by this division. In most northern
flounders the eyes are on the right side in the adult, in tropical
forms more frequently on the left, these distinctions corresponding
with others in the structure of the fish.
In the lowest of the fish-like forms, the lancelet, the eye is simply
a minute pigment-spot situated in the anterior wall of the ventricle
at the anterior end of the central nervous system. In the hagfishes,
which stand next highest in the series, the eye, still incomplete, is
very small and hidden by the skin and muscles. This condition is very
different from that of the blind fishes of the higher groups, in which
the eye is lost through atrophy, because in life in caves or under
rocks the function of seeing is no longer necessary.
=The Organs of Hearing.=--The ear of the typical fish consists of the
labyrinth only, including the vestibule and usually three semicircular
canals, these dilating into sacs which contain one or more large,
loose bones, the ear-stones or otoliths. In the lampreys there are two
semicircular canals, in the hagfish but one. There is no external ear,
no tympanum, and no Eustachian tube. The ear-sac on each side is lodged
in the skull or at the base of the cranial cavity. It is externally
surrounded by bone or cartilage, but sometimes it lies near a
fontanelle or opening in the skull above. In some fishes it is brought
into very close connection with the anterior end of the air-bladder.
The latter organ it is thought may form part of the apparatus for
hearing. The arrangement for this purpose is especially elaborate
in the carp and the catfish families. In these fishes and their
relatives (called _Ostariophysi_) the two vestibules are joined in a
median sac (_sinus impar_) in the substance of the basioccipital. This
communicates with two cavities in the atlas, which again are supported
by two small bones, these resting on a larger one in connection with
the front of the air-bladder. The system of bones is analogous to that
found in the higher vertebrates, but it connects with the air-bladder,
not with an external tympanum. The bones are not homologous with those
of the ear of higher animals, being processes of the anterior vertebræ.
The tympanic chain of higher vertebrates has been thought homologous
with the suspensory of the mandible.
[Illustration: FIG. 88.--Brook Lamprey, _Lampetra wilderi_ Jordan and
Evermann. (After Gage.) Cayuga Lake.]
The otoliths, commonly two in each labyrinth, are usually large, firm,
calcareous bodies, with enamelled surface and peculiar grooves and
markings. Each species has its own form of otolith, but they vary much
in different groups of fishes.
[Illustration: FIG. 89.--European Lancelet, _Branchiostoma lanceolatum_
(Pallas). (After Parker and Haswell.)]
In the Elasmobranchs (sharks and rays) and in the Dipnoans the ear-sac
is enclosed in the cartilaginous substance of the skull. There is a
small canal extending to the surface of the skull, ending sometimes in
a minute foramen. The otoliths in these fishes are soft and chalk-like.
The lancelet shows no trace of an ear. In the cyclostomes, hagfishes,
and lampreys it forms a capsule of relatively simple structure
conspicuous in the prepared skeleton.
The sense of hearing in fishes cannot be very acute, and is at the
most confined to the perception of disturbances in the water. Most
movements of the fish are governed by sight rather than by sound. It
is in fact extremely doubtful whether fishes really hear at all, in
a way comparable to the auditory sense in higher vertebrates. Recent
experiments of Professor G. H. Parker on the killifish tend to show a
moderate degree of auditory sense which grades into the sense of touch,
the tubes of the lateral line assisting in both hearing and touch.
While the killifish responds to a bass-viol string, there may be some
fishes wholly deaf.
=Voices of Fishes.=--Some fishes make distinct noises variously
described as quivering, grunting, grating, or singing. The name
grunt is applied to species of _Hæmulon_ and related genera, and
fairly describes the sound these fishes make. The Spanish name ronco
or roncador (grunter or snorer) is applied to several fishes, both
sciænoid and hæmuloid. The noise made by these fishes may be produced
by forcing air from part to part of the complex air-bladder, or it may
be due to grating one on another of the large pharyngeals. The grating
sounds arise, no doubt, from the pharyngeals, while the quivering or
singing sounds arise in the air-bladder. The midshipman, _Porichthys
notatus_, is often called singing fish, from a peculiar sound it emits.
These sounds have not yet been carefully investigated.
=The Sense of Taste.=--It is not certain that fishes possess a sense of
taste, and it is attributed to them only through their homology with
the higher animals. The tongue is without delicate membranes or power
of motion. In some fishes certain parts of the palate or pharyngeal
region are well supplied with nerves, but no direct evidence exists
that these have a function of discrimination among foods. Fishes
swallow their food very rapidly, often whole, and mastication, when it
takes place, is a crushing or cutting process, not one likely to be
affected by the taste of the food.
=The Sense of Touch.=--The sense of touch is better developed among
fishes. Most of them flee from contact with actively moving objects.
Many fishes use sensitive structures as a means of exploring the bottom
or of feeling their way to their food. The barbel or fleshy filament
wherever developed is an organ of touch. In some fishes, barbels are
outgrowths from the nostrils. In the catfish the principal barbel grows
from the rudimentary maxillary bone. In the horned dace and gudgeon the
little barbel is attached to the maxillary. In other fishes barbels
grow from the skin of the chin or snout. In the goatfish and surmullet
the two chin barbels are highly specialized. In _Polymixia_ the chin
barbels are modified _branchiostegals_. In the codfish the single
beard is little developed. In the gurnards and related forms the lower
rays of the pectoral are separate and barbel-like. Detached rays of
this sort are found in the thread-fins (_Polynemidæ_), the gurnards
(_Triglidæ_), and in various other fishes. Barbels or fleshy flaps are
often developed over the eyes and sometimes on the scales or the fins.
[Illustration: FIG. 90.--Goatfish, _Pseudupeneus maculatus_ (Bloch).
Woods Hole.]
The structure of the lateral line and its probable relation as a
sense-organ is discussed on page 23. It is probable that it is
associated with sense of touch, and hearing as well, the internal ear
being originally "a modified part of the lateral-line system," as shown
by Parker,[8] who calls the skin the lateral line and the ear "three
generations of sense-organs."
The sense of pain is very feeble among fishes. A trout has been known
to bite at its own eye placed on a hook, and similar insensibility
has been noted in the pike and other fishes. "The Greenland shark,
when feeding on the carcass of a whale, allows itself to be repeatedly
stabbed in the head without abandoning its prey." (GÜNTHER.)
FOOTNOTES:
[8] See Parker, on the sense of hearing in fishes, American Naturalist
for March, 1903.