636 ICHTHYOLOGY [EXTERNAL axis, is found. This deviation from the typical form may proceed so far that the vertical axis greatly exceeds the longitudinal in length ; generally all the parts of the body participate in this form, but in one kind of fish (the Ortha- goriscus or sun-fish) it is chiefly the tail that has been shortened, that being reduced so much as to present the ap pearance of being cut off. An excessive lengthening of the longitudinal axis, with a shortening of the vertical, occurs in eels and eel-like fishes, and in the so-called band-fishes. They are bottom-fish, capable of insinuating themselves into narrow crevices and holes. The form of the body in these long fishes is either cylindrical (snake-like), as in the eels and many codfishes, or strongly compressed, as in the band-fishes (Trichiurus, Regalecus, &c.). It is chiefly the tail that is lengthened, but frequently the head and trunk partici pate more or less in this form. Every possible variation occurs between these and other principal types of form. The old ichthyologists, even down to Linnyeus, depended in great measure on them for classification ; but, although the same form of body often obtains in the same groups of fishes, similarity of form by no means indicates natural affinity ; it only indicates similarity of habits and mode of life. Head. The External Parts of the Head. The eye divides the head into the ante-orbital and post-orbital portions. In most fishes, especially in those with a compressed head, it is situated on the side and in the anterior half of the length of the head; in many others, chiefly those with a de pressed head, it is directed upwards, and sometimes situated quits at the upper side ; in a very few, the eyes look obliquely downwards. In ths flat-fishes both eyes ars on the same side of the head, either the right or the left, always on that which is directed towards the light, and coloured. Fishes in general, as compared with other Vertebrata, have large eyes. Sometimes these organs are enormously enlarged, indicating either that the fish is nocturnal in its habits, or lives at a depth to which only a part of the sun s rays penetrate. On the other hand, small eyes occur in fishes inhabiting muddy places, or great depths, to which scarcely any light descends, or in fishes in which the want of an organ of sight is compensated by the development of other organs of sense. In a few fishes, more particularly those inhabiting caves or the greatest depths of the ocean, the eyes have become quite rudimentary and hidden under the skin. In the ante-orbitil portion of the .head, or the snout, are situated the mouth and nostrils. Month. The mouth is formed by the intermaxillary and maxillary bones or the intermaxillary only in the upper jaw, and by the mandibular bjne in the lower. These bones are either bare or covered by integument, to which frequently labial folds or lips are added. As regards form, the mouth offers as many variations as the body itself, according to the nature of the food, and the mode of feeding. It may be narrow, or extremely wide and cleft nearly to the hind margin of the head ; it may be semi-elliptical, semi circular, or straight in a transverse line; it may be quite in front of the snout (anterior), on its upper surface (superior), on its lower (inferior), or extending along each side (lateral) ; sometimes it is subcircular, organized for sucking. The jaws of some fishes are modified into a special weapon of attack (sword-fish, saw -fish); in fact, throughout the whole class of fishes the jaws are the only organ ever specialized for this purpose, weapons on other parts of the body being purely defensive. Both jaws may be provided with skinny appendages, barbels, which, if developed and movable, are sensitive organs of touch. Nostrils. In the majority of fishes the nostrils exhibit a double opening on each side of the upper surface of the snout, the openings of each side being more or less close together. They lead into a shallow groove, and only in one family (the Myxinoids) perforate the palate. In this family, as well as in the lampreys, the nasal aperture is single. In Fin. 2. Confluent nasal and buccal cavities of ChiloscylliuiH trispeculare; toutli of the natural size. FIG. 1. Head of Afordaaa mordax, showing the single nobtril and seven branchial openings. many eels the openings are lateral, the lower perforating, the upper lip. In the sharks and rays (figs. 2, 3, 4) they are at the lower surface of the snout, and more or less confluent. And, finally, in the Dipnoi (fig. 35) and other Ga noids, one at least is within the labial bound ary of the mouth. The space across the forehead, between the orbits, is called the inter- orbital space; that below the orbits, the infraorbi- tal or suborbital region. In the postorbital part of the head there are distinguished, at least in most Teleosteous fishes and many Ganoids, the pneoperculum, a sub-semicircular bone, gene rally with a free and often serrated or vari ously-armed margin ; the operculum, form ing the posterior mar gin of the gill-open ing; and the suboper- culum and interoper- culum along its in ferior margin. All these bones, collec tively called opercula, form the gill-cover, a thin bony lamella covering the cavity containing the gills. The gill-opening is a foramen, or slit, be hind or below the head, by which the water that has been taken up through the mouth for the purpose of breathing is again expelled. This slit may extend from the upper end of the operculum all round the side of the head to the symphysis of the lower jaw; or it maybe shortened and finally reduced to a small opening on any part of the mar gin of the gill-cover. Sometimes (/Si/mbranckus ) the two openings, thus reduced, coalesce, and form what externally appears as a single opening only. The margin of the gill- cover is provided with a cutaneous fringe, in order more effectually to close the gill-opening; and this fringe is sup ported by one or several or many bony rays, the branch io- stegals. The space on the chest between the two rami of the lower jaw and between the gill-openings is called the isthmus. The sharks and rays differ from the Teleosteoxis and Ganoid fishes in having five branchial slits (six or seven in Hexanchus and Heptanchus), which are lateral in the sharks, and at the. lower surface of the head in the rays FIG. 3. Nostrils of Raia lemprieri, with nasal Gills
flaps reverted.
