The ribs of a bony fish are attached to the transverse processes, or to the bodies of the abdominal vertebrae, in the form of slender curved bones which articulate with no more than one vertebra each, and that only at a single point. Unlike the ribs of the higher Vertebrates, the ribs do not enclose a thoracic cavity, but are simply embedded in the muscles which bound the abdomen. Usually each rib gives off a spine-like bone, which is directed backwards amongst the muscles. Inferiorly the extremities of the ribs are free, or are rarely united to dermal ossifications in the middle line of the abdomen; but there is never any breast-bone or sternum properly so called.
The only remaining bones connected with the skeleton of the trunk are the so-called interspinous bones (fig. 248, i i).
These form a series of dagger-shaped bones, plunged in the middle line of the body between the great lateral muscles which make up the greater part of the body of a fish. The internal ends or points of the interspinous bones are attached by ligament to the spinous processes of the vertebrae; whilst to their outer ends are articulated the "rays" of the so-called "median" fins, which will be hereafter described. As a rule, there is only one interspinous bone to each spinous process, but in the Flat-fishes (Sole, Turbot, etc.) there are two.
Besides the fins which represent the limbs (pectoral and ventral fins), fishes possess other fins placed in the middle line of the body, and all of these alike are supported by bony spines or "rays," which are of two kinds, termed respectively "spinous rays" and "soft rays." The "spinous rays" are simple bony spines, apparently composed of a single piece each, but really consisting of two halves firmly united along the middle line. The "soft rays" are composed of several slender spines proceeding from a common base, and all divided transversely into numerous short pieces. The soft rays occur in many fishes in different fins, but they are invariably found in the caudal fin or tail (fig. 248, c). The rays of the median fins, whatever their character may be, always articulate by a hinge-joint with the heads of the interspinous bones.
The skull of the bony fishes is an extremely complicated structure, and it is impossible to enter into its composition here. The only portions of the skull which require special mention are the bones which form the gill-cover or operculum, and the hyoid bone with its appendages. For reasons connected with the respiratory process in fishes, as will be afterwards seen, there generally exists between the head and the scapular arch a great cavity or gap on each side, within which are contained the branchiae. The cavity thus formed opens externally on each side of the neck by a single vertical fissure or "gill-slit," closed by a broad flap, called the "gill-cover" or "operculum," and by a membrane termed the "branchiostegal membrane."
The gill-cover (fig. 249, p, o, s, i) is composed of a chain of broad flat bones, termed the opercular bones. Of these, the innermost articulates with the skull (tympano-mandibular arch), and is called the "prae-operculum; " the next is a large bone called the "operculum "proper; and the remaining two bones, called respectively the "sub-operculum " and "inter-oper-culum," form, with the operculum proper, the edge of the gill-cover. These various bones are united together by membrane, and they form collectively a kind of movable door, by means of which the branchial chamber can be alternately opened and shut. Besides the gill-cover, however, the branchial chamber is closed by a membrane called the "branchiostegal membrane," which is attached to the os hyoides. The membrane is supported and spread out by a number of slender curved spines, which are attached to the lateral branches of the hyoid bone, act very much as the ribs of an umbrella, and are known as the "branchiostegal rays" (fig. 249, d). The hyoid arch of fishes is attached to the temporal bones of the skull by means of two slender styliform bones, which correspond to the styloid processes of man, and are called the " stylohyal " bones (fig. 250, f). The rest of the hyoid arch is composed of a central portion and two lateral branches. Each branch is composed of the following parts : I. A triangular bone attached above to the stylohyal, and termed the "epihyal bone" (fig. 250, e); 2. A much longer bone, known as the "ceratohyal" (d). The central portion of the hyoid arch is made up of two small polyhedral bones - the "basihyals" (b). From the basihyal there extends forwards in many fishes a slender bone, which supports the tongue, and is termed the "glossohyal" or "lingual" (a). There is also another compressed bone which extends backwards from the basihyals, and which is known as the "urohyal bone" (c). This last-mentioned bone is of importance, as it often extends backwards to the point of union of the cora-coid bones, and thus forms the isthmus which separates the two branchial apertures.
Fig. 249. - Skull of Cod (Morrhua vulgaris) - Cuvier. a Urohyal; b Basihyal; c Ceratohyal; d Branchiostegal rays ; p Pra-operculum; o Operculum proper; s Sub-operculum; i Inter-operculum ; m Mandible ; n Inter-maxillary bone.
From the outer margins of the epihyal and ceratohyal bones on each side arise the slender curved "branchiostegal rays," which have been previously mentioned. There are usually seven of these on each side. Above the urohyal, and attached in front to the body of the os hyoides, is a chain of bones, placed one behind the other, and termed by Owen the "basibran-chial bones." Springing from these are four bony arches - the "branchial arches" - which proceed upwards to be connected superiorly by ligament with the under surface of the skull. The branchial arches - as will be subsequently described - carry the branchiae, and each is composed of two main pieces, termed respectively the "cerato-branchial" and "epibranchial" bones. The second and third arches are connected with the skull by the intervention of two small bones, often called the "superior pharyngeal bones," but termed by Owen the "pharyngo - branchial" bones.