THE ORGANS OF SENSE.
Characteristic of the _Organ of Smell_ in Fishes is that it has
no relation whatever to the respiratory function, with the exception
of the _Dipnoi_, in which possibly part of the water received for
respiration passes through the nasal sac.
The olfactory organ is single in _Branchiostoma_ and the _Cyclostomes_.
In the former a small depression on the front end of the body, clothed
with a ciliated epithelium, is regarded as a rudimentary organ of
smell. In the adult _Petromyzon_ a membranous tube leads from the
single opening on the top of the head into the cartilaginous olfactory
capsule, the inside of which is clothed by membranes prolonged into a
posterior blind tube (Fig. 30, _s_), which penetrates the cartilaginous
roof of the palate, but not the mucous membrane of the buccal cavity.
In the Myxinoids the outer tube is strengthened by cartilaginous
rings like a trachea; the capsule is lined by a longitudinally folded
pituitary membrane, and the posterior tube opens backwards on the roof
of the mouth; the opening is provided with a valve.
In all other Fishes the organ of smell is double, one being on each
side; it consists of a sac lined with a pituitary membrane, and
without, or with one or two, openings. The position of these openings
is very different in the various orders or suborders of Fishes.
In the _Dipnoi_ the nasal sac opens downwards by two wide openings
which are within the boundaries of the cavity of the mouth. The
pituitary membrane is transversely folded, the transverse folds being
divided by one longitudinal fold. The walls of the sac are strengthened
by sundry small cartilages.
Also in _Chondropterygians_ the openings, of which there is one
to each sac, are on the lower part of the snout, and in the Rays,
Holocephali, and some Sharks, each extends into the cleft of the mouth.
The openings are protected by valvular flaps, supported by small
cartilages, and moved by muscles, whence it may be concluded that these
fishes are able to scent (actively) as well as to smell (passively).
[Illustration: Fig. 47.--Nostrils of _Raia lemprieri_, with
nasal flaps reverted.]
In the majority of _Teleostei_ the olfactory capsules are lateral or
superior on the snout, covered externally by the skin, each usually
pierced by two openings, which are either close together, or more
or less remote from each other; the posterior is generally open,
the anterior provided with a valve or tube. In the _Chromides_ and
_Labroidei ctenoidei_ a single opening only exists for each sac. In
the _Murænidæ_ the two openings of each side are either superior, or
lateral, or labial, that is, they are continued downwards and pierce
the margin of the upper lip. In many Tetrodonts nasal openings are
absent, and replaced by a conical papilla, in which the olfactory nerve
terminates.
It is certain that fishes possess the faculty of perceiving odours,
and that various scents attract or repel them. A mangled carcase
or fresh blood attracts Sharks as well as the voracious Serrasal
monoids of the South American rivers. There is no reason to doubt that
the seat of that perception is in the olfactory sac; and it may be
reasonably conjectured that its strength depends mainly on the degree
of development indicated by the number and extent of the interior folds
of the pituitary membrane.
_Organ of Sight_.--The position, direction, and dimensions of the
eyes of fishes vary greatly. In some they have an upward aspect, and
are often very close together; in others they are lateral, and in a
few they are even directed downwards. The Flat-fishes represent the
extraordinary anomaly that both eyes are on the same side of the head,
and rarely on the same level, one being generally placed more forward
than the other. In certain species of marine fishes the eyes are of
an extraordinary size, a peculiarity indicating that the fish either
lives at a great depth, to which only a small proportion of the rays
of light penetrate, or that it is of nocturnal habits. In fishes which
have descended to such great depths that no rays whatever can reach
them, or in freshwater fishes living in caves, or in species which
grovel and live constantly in mud, the eyes are more or less aborted,
sometimes quite rudimentary, and covered by the skin. In very few this
organ appears to be entirely absent. In some Gobioids and Trachinoids
(_Periophthalmus_, _Boleophthalmus_, _Uronoscopus_, etc.) the eyes,
which are on the upper side of the head, can be elevated and depressed
at the will of the fish. In the range of their vision and acuteness of
sight, Fishes are very inferior to the higher classes of Vertebrates,
yet at the same time it is evident that they perceive their prey or
approaching danger from a considerable distance; and it would appear
that the visual powers of a _Periophthalmus_, when hunting insects on
mud-flats of the tropical coasts, are quite equal to that of a frog.
Again, the discrimination with which fishes sometimes prefer one colour
or kind of artificial fly to another affords sufficient evidence
that the vision, at least of certain species is by no means devoid of
clearness and precision.
The eye of _Branchiostoma_ is of the most rudimentary condition. It is
simply a minute speck coated by dark pigment, and receiving the end of
a short nerve. In _Myxinoids_ the minute rudiment of the eye is covered
by the skin and muscles. This is also the case in many of the blind
Teleosteous fishes; however, whilst in the former fishes the organ of
sight has not attained to any degree of development, the rudimentary
eye of blind Teleostei is a retrogressive formation, in which often
a lens and other portions of the eye can be recognised. In fishes
with a well-developed eye it is imbedded in a layer of gelatinous and
adipose substance, which covers the cavity of the orbit. A lacrymal
gland is absent. In the orbit of one fish only, _Chorismodentex_, an
organ has been found which can be compared to a _saccus lacrymalis_.
It is a round, blind, wide sac, of the size of a pea, situated below
the anterior corner of the orbit, between the maxillary bone and the
muscles of the cheek, communicating by a rather wide foramen with
the orbital cavity. The membrane by which it is formed is continuous
with that coating the orbita. In the Chondropterygians the eyeball is
supported by and moves on a cartilaginous peduncle of the orbital wall.
In the majority of Teleosteans, and in Acipenser, a fibrous ligament
attaches the sclerotic to the wall of the orbit. The proper muscles
of the eyeball exist in all fishes, and consist of the four _Musculi
recti_ and the two _M. obliqui_. In many Teleostei the former rise
from a subcranial canal, the origin of the _M. rectus externus_ being
prolonged farthest backwards. The _Recti_ muscles are extraordinarily
long in the Hammerheaded Sharks, in which they extend from the basis
cranii along the lateral prolongations of the head to the eyes, which
are situated at the extremities of the hammer.
In all fishes the general integument of the head passes over the
eye, and becomes transparent where it enters the orbit; sometimes it
simply passes over the orbit, sometimes it forms a circular fold. The
anterior and posterior portions may be especially broad and the seat
of an adipose deposit (_adipose eyelids_), as in _Scomber_, _Caranx_,
_Mugil_, etc. In many of these fishes the extent of these eyelids
varies with the seasons; during the spawning season they are so much
loaded with fat as nearly to hide the whole eye. Many Sharks possess a
_nictitating_ membrane, developed from the lower part of the palpebral
fold, and moved by a proper set of muscles.
[Illustration: Fig. 48.
Vertical section through eye of Xiphias. (After Owen.)
_co_, Cornea; _sc_, sclerotica; _o_, nervus opticus; _c_,
sclerotic capsule; _a_, membrana argentea; _v_, membrana
vasculosa; _u_, membrana uvea; _ch_, choroid gland; _r_,
retina; _f_, processus falciformis; _h_, humor vitreus; _l_,
lens; _i_, iris.]
The form of the _bulbus_ (Fig. 48) is subhemispherical, the cornea
(_co_) being flat. If it were convex, as in higher Vertebrates, it
would be more liable to injury; but being level with the side of the
head the chances of injury by friction are diminished. The _sclerotica_
(_sc_) is cartilaginous in Chondropterygians and Acipensers, fibrous
and of varying thickness in Teleosteans, in the majority of which it
is supported by a pair of cartilaginous or ossified hemispheroid cups
(_c_). In a few fishes, as in _Ceratodus_, _Xiphias_, the cups are
confluent into one cup, which possesses a foramen behind to allow the
passage of the optic nerve (_o_). The _cornea_ of _Anableps_ shows an
unique peculiarity. It is crossed by a dark horizontal stripe of the
conjunctiva, dividing it into an upper and lower portion; also the iris
is perforated by two pupils. This fish is observed to swim frequently
with half of its head out of the water, and it is a fact that it can
see out of the water as well as in it.
The membranes situated between the sclerotica and retina are
collectively called _choroidea_, and three in number. The one in
immediate contact with the sclerotic, and continued upon the iris,
is by no means constantly present; it is the _membrana argentea_
(_a_), and composed of microscopical crystals reflecting a silvery or
sometimes golden lustre. The middle layer is the _membrana vasculosa_
s. _halleri_ (_v_), the chief seat of the ramifications of the choroid
vessels; the innermost layer is the _membrana ruyscheana_ or _uvea_
(_u_), which is composed of hexagonal pigment-cells, usually of a deep
brown or black colour.
In many _Teleostei_ a _rete mirabile_ surrounds the entry of the optic
nerve; it is situated between the membrana argentea and vasculosa, and
called the _choroid gland_ (_ch_). It receives its arterial blood from
the artery issuing from the pseudobranchia; the presence of a choroid
gland always being combined with that of a pseudobranchia. Teleosteans
without pseudobranchia lack a choroid gland. In the Palæichthyes, on
the other hand, the pseudobranchia is present and a choroid gland
absent.
The _iris_ (_i_) is merely the continuation of the choroid membrane;
its capability of contracting and expanding is much more limited than
in higher Vertebrates. The _pupil_ is generally round, sometimes
horizontally or vertically elliptical, sometimes fringed. In the
Rays and Pleuronectidæ a lobe descends from the upper margin of the
pupil, and the outer integument overlying this lobe is coloured and
non-transparent; a structure evidently preventing light from entering
the eye from above.
In most _Teleostei_ a fold of the _Choroidea_, called the _Processus
falciformis_ (_f_), extends from the vicinity of the entrance of the
optic nerve to the lens. It seems to be constantly absent in Ganoids.
The _retina_ (_r_) is the membrane into which the optic nerve
penetrates, and in which its terminal filaments are distributed. It
consists of several layers (Fig. 49). The outermost is an extremely
delicate membrane (_a_), followed by a layer of nerve-cells (_b_), from
which the terminal filaments issue, passing through several granular
strata (_c_, _d_, _e_), on which the innermost stratum rests. This
stratum is composed of cylindrical rods (_f_) vertically arranged,
between which twin fusiform corpuscles (_g_) are intercalated. This
last layer is thickly covered with a dark pigment. The retina extends
over a portion of the iris, and a well-defined raised rim runs along
its anterior margin.
[Illustration: Fig. 49.--Vertical section of the Retina of the
Perch, magn. X 350.]
The _vitreous humour_ (Fig. 48, _h_) which fills the posterior
cavity of the eyeball, is of a firmer consistency than in the higher
Vertebrates. The _lens_ is spherical, or nearly so; firm, denser
towards the centre, and lies in a hollow of the vitreous humour. When
a falciform process is present, it is with one end attached to the
lens, which is thus steadied in its position. It consists of concentric
layers consisting of fibres, which in the nucleus of the body have
marginal teeth, by which they are interlocked together. In _Petromyzon_
this serrature is absent, or but faintly indicated.
[Illustration: Fig. 50.--Interlocking fibres of lens, highly
magnified.]
The anterior cavity of the eye is very small in Fishes, in consequence
of the small degree of convexity of the cornea; the quantity of the
aqueous humour, therefore, is very small, just sufficient to float the
free border of the iris; and the lessened refractive power of the
aqueous humour is compensated by the greater convexity of the lens.
* * * * *
_Organ of Hearing._--No trace of an organ of hearing has been found
in _Branchiostoma_. In the _Cyclostomes_ the labyrinth is enclosed in
externally visible cartilaginous capsules laterally attached to the
skull; it consists of a single _semicircular canal_ in the Myxinoids,
whilst the Petromyzontes possess two semicircular canals with a
_vestibulum_.
In all other fishes the labyrinth consists of a vestibule and three
semicircular canals, the vestibule dilating into one or more sacs which
contain the otoliths. A tympanum, tympanic cavity, and external parts,
are entirely absent in the class of fishes.
In the _Chondropterygians_ and _Dipnoi_, the labyrinth is enclosed in
the cartilaginous substance of the skull. In the former the excavation
in the cartilage is larger than the membranous labyrinth, but nearly
corresponds to it in form; the part which receives the membranous
vestibulum is called _Vestibulum cartilagineum_, from which a canal
issues and penetrates to the surface of the skull, where it is closed
by the skin in Sharks, but opens by a minute foramen in Rays. The
otolithic contents are soft and chalklike.
In the _Holocephali_ part of the labyrinth is enclosed in the cartilage
of the skull, another part being in the cranial cavity, as in Ganoids
and Teleosteans. The membranous vestibulum is continued by a canal to a
single opening in the roof of the skull, from which two smaller canals
are continued to two small foramina in the skin covering the occipital
region.
In the _Teleosteans_ the sac which contains the otoliths lies on each
side of the base of the cranial cavity and is often divided by a septum
into two compartments of unequal size, each containing a firm and solid
_otolith_; these bodies (Fig. 51), possess indented margins, frequently
other impressions and grooves, in which nerves from the N. acusticus
are lodged; they vary much in size and form, but in both respects show
a remarkable constancy in the same kind of fishes. The vestibule is
outwards in contact with the osseous side wall of the skull, inwards
with the metencephalon and medulla oblongata; it contains another firm
concretion, and opens by five foramina into the three semicircular
canals. The terminations of the acustic nerve are distributed over the
vestibular concretion and the ampulliform ends (Fig. 52 _p_) of the
semicircular canals, without being continued into the latter, which are
filled with fluid. The semicircular canals (Fig. 52 _g_), are sometimes
lodged in the cranial bones, sometimes partly free in the cranial
cavity. Many Teleostei have fontanelles in the roof of the skull,
closed by skin or very thin bone only at the place where the auditory
organ approaches the surface, by which means sonorous undulations must
be conducted with greater ease to the ear.
[Illustration:
Fig. 51.--Otolith of Haddock (Gadus æglefinus). I. Outer, II.
Inner aspect.]
In many Teleostei a most remarkable relation obtains between the organ
of hearing and the air-bladder. In the most simple form this connection
is established in Percoids and the allied families, in which the two
anterior horns of the air-bladder are attached to fontanelles of the
occipital region of the skull, the vestibulum occupying the opposite
side of the membrane by which the fontanelle is closed. The condition
is similar, but more complicated in many Clupeoids. The anterior narrow
end of the air-bladder is produced into a canal at the base of the
skull, and divided into two very narrow branches, which again bifurcate
and terminate in a globular swelling. An appendage of the vestibulum
meets the anterior of these swellings, and comes into close contact
with it. Besides, the two vestibules communicate with each other by a
transverse canal, crossing the cranial cavity below the brain.
[Illustration: Fig. 52.--Communication between auditory organ and
air-bladder in the Carp. (After E. H. Weber.)
_a_, Basisphenoid; _b_, Occipital; _c_, Supraoccipital;
_d_, Exoccipital; _e_, Paroccipital; _f_, Alisphenoid; _g_,
Neural arch of first vertebra; _h_, _i_, _k_, second, third,
and fourth vertebra; _h_’, _i_’, Parapophyses of second and
third vertebra; _i_", process of the third vertebra for
the attachment of the air-bladder; _k_, _l_, _m_, Chain
of ossicles; _n_, Air-bladder; _o_, vestibulum; _p_, _p_,
Ampullæ; _q_, _q_, Canales semicirculares; _r_, Sinus impar.]
The connection is effected by means of a chain of ossicles in
_Siluridæ_, _Characinidæ_, _Cyprinidæ_ and _Gymnotidæ_. A canal issues
from the communication between vestibule and its sac, and meeting that
from the other side forms with it a common _sinus impar_ (Fig. 52,
_r_), lodged in the substance of the basi-occipital; this communicates
on each side by a small orifice with two subspherical atria, on
the body of the atlas, close to the foramen magnum. Each atrium is
supported externally by a small bone (_m_); a third larger bone (_k_)
completes the communication with the anterior part of the air-bladder.
From the sinus impar a bifid canal penetrates into the alisphenoids,
in which it terminates. In _Cobitis_ and several Loach-like Siluroids
the small air-bladder consists of two globular portions placed side
by side, and wholly included within two bullæ, formed by the modified
parapophyses of the second and third vertebræ. The three ossicles on
each side are present, but concealed by the fore part of the osseous
bulla.
* * * * *
_Organ of Taste._--Some fishes, especially vegetable feeders, or those
provided with broad molar-like teeth, masticate their food; and it
may be observed in Carps and other Cyprinoid fish, that this process
of mastication frequently takes some time. But the majority of fish
swallow their food rapidly, and without mastication, and therefore we
may conclude that the sense of taste cannot be acute. The tongue is
often entirely absent, and even when it exists in its most distinct
state, it consists merely of ligamentous or cellular substance, and
is never furnished with muscles capable of producing the movements of
extension or retraction as in most higher Vertebrates. A peculiar organ
on the roof of the palate of Cyprinoids, is perhaps an organ adapted
for perception of this sense; in these fishes the palate between and
below the upper pharyngeal bones is cushioned with a thick, soft
contractile substance, richly supplied with nerves from the Nervi vagus
and glossopharyngeus.
_Organs of Touch._--The faculty of touch is more developed than that of
taste, and there are numerous fishes which possess special organs of
touch. Most fishes are very sensitive to external touch, although their
body may be protected by hard horny scales. They perceive impressions
even on those parts which are covered by osseous scutes, in the same
manner as a tortoise perceives the slightest touch of its carapace. The
seat of the greatest sensitiveness, however, appears to be the snout
and the labial folds surrounding the mouth. Many species possess soft
and delicate appendages, called barbels, which are almost constantly
in action, and clearly used as organs of touch. Among the _Triglidæ_
and allied families, there are many species which have one or more
rays of the pectoral fin detached from the membrane, and supplied with
strong nerves. Such detached rays (also found in the _Polynemidæ_,
_Bathypterois_) are used partly for locomotion, partly for the purpose
of exploring the ground over which the fish moves.
Some fish appear to be much less sensitive than others, or at least
lose their sensitiveness under peculiar circumstances. It is well known
that a Pike, whose mouth has been lacerated and torn by the hook,
continues to yield to the temptation of a bait immediately afterwards.
The Greenland Shark when feeding on the carcass of a whale allows
itself to be repeatedly stabbed in the head without abandoning its
prey. A pair of Congers are so dead to external impression at the time
of copulation, and so automatically, as it were, engaged, that they
have been taken by the hand together out of the water.