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.
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.