Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/670

652 652 ICHTHYOLOGY [ORGANS OF SENSE. ORGANS OF SENSE. Organ of Smell. It is characteristic of the organ of smell in fishes that it has no relation whatever to the re spiratory 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 re garded 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 olfac tory capsule, the inside of which is clothed by membranes prolonged into a posterior blind tube (fig. 31, s), which penetrates the cartilaginous roof of the palate, but not the mucous membrane of the buccal cafity. 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 being provided with a valve, In all other fishes the organ of smell is double, there being one on each side ; it consists of a sac lined with a pituitary membrane, and may be provided with one or with two openings, or may have none. The position of these openings is very different in the various orders and suborders of fishes. 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 Serrasalmonoids 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 dimen sions 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. In a very few this organ appears to be entirely absent. In some Gobioids and Trachinoids (Periophthal- mus, Boleophthalmus, Uranoscopus, &c.) the eyes, which are on the upper side of the head, can be elevated and de pressed at the will of the fish. In the range of their vision and in their acuteness of sight, fishes are very in ferior to the higher classes of vertebrates, yet at the same time it is evident that they perceive their prey, or approach ing danger, from a considerable distance ; and it would appear that the visual powers of a Periopliihalmns (fig. 12), when hunting insects on the mud-flats of the tropical coasts, are quite equal to those of a frog. The eye of Branchiostoma (fig. 30, g) is of the most rudi mentary kind. It is simply a minute speck coated with a dark pigment, and receiving the end of a short nerve. In Myxinoids the minute rudiment of the eye is covered by skin and muscles. This is also the case in many of the blind Teleosteous fishes ; whilst in the former fishes, however, the organ of sight has not attained to any degree of development, the rudimentary eye of blind Teleostei is a retrogressive formation, in which a lens and other portions of the eye can often be recognized. 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 lachrymal gland is absent. 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 are always present. In all fishes the general integument through eye of Xiphias. (After Owen.) co, cornea ; sc, sclerotica ; o, nervus optieus ; c,sclerotic capsule; a, membrana argrntea; v, membrana vasculosa ; , membrana uvea ; ch, cho- roid gland; r, retina; /, processus falciformis; h, humor vitreus; /, lens; f, iris. 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 specially broad, becoming the seat of an adipose deposit (adipose eyelids), as in Scomber, Caranx, Mugil, &c. In many of these fishes the extent of the 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. The form of tlie bulbus (fig. 43) is subhemispherical, the cornea (co) being flat. If it were convex, as in higher vertebrates, it would be more liable to injury ; but, as it is level with the side of the head, the chances of injury by friction are diminished. The sclerotica (sc) is cartilaginous in Chon dropterygians and Acipensers, fibrous and of varying thickness in Teleosteans, in the majority of which it is supported by a pair of cartilaginous or ossified hemi- spheroid 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 FlG 43 _ VertIcal 8ec t ion nerve (o). The membranes situated between the sclerotica and retina are collectively called choroidea, and are three in number. The one in immediate contact with the sclero tic, and continued upon the iris, is by no means constantly present ; it is the mem brana argentea (a), and is composed of mi croscopical crystals reflecting a silvery or sometimes golden lustre. The middle layer is the membrana vas culosa or 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 being always combined with that of a pseudobranchia. Teleosteans without pseudobranchia have no choroid gland. In the Palceichthyes, 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, some times horizontally or vertically elliptical, sometimes fringed. In the rays and Plcuroncctulce 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 vitreous humour (h), which fills the posterior cavity of the eye-ball, is of a firmer consistency than in the higher verte brates. 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 has one end attached to the lens, which is thus steadied in its position. It is composed of concentric layers consisting of fibres, which in the nucleus of the body have marginal teeth, by which they are interlocked together. In Pctro- myzon this scrrature is absent, or but faintly indicated. The anterior cavity of the eye is very small in fishes, in conse quence of the small degree of convexity in 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