CHAPTER VII.. The Organs of Sense.

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).

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.

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.

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.

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.

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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.

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.

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.

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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.