Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/661

643 OSTEOLOGY.] ICHTHYOLOGY 043 are attached by a layer of ligameutous and cellular tissue. Each of the first three branchial arches consists of four pieces movably connected with one another. The lowest is the hypobrauchial (57), the next, a much longer one, the FIG. 28. Hyoid bone of the Perch. ceratobranchial (58), and, above this, a slender and short irregularly-shaped epibranchial. In the fourth arch the hypobranchial is absent. The uppermost of these segments, especially of the fourth arch, are dilated, and more or less confluent; they are beset with fine teeth, and generally distinguished as the upper pharyngeal bones. Only the ceratobranchial is represented in the fifth arch or lower pharyngeal. On their outer convex side the branchial segments are grooved for the reception of large blood vessels and nerves ; on the inner side they support horny processes, called the gill-rakers, which do not form part of the skeleton. 6. Membrane-Bones of the Respiratory Portion of the Visceral Skeleton of the Skull. These include the oper- cular pieces, viz., the operculum (28), suboperculum (32), and interoperculum (33). The last of these is the least constant ; it may be entirely absent, and represented by a ligament extending from the mandible to the hyoid. The operculum is the principal piece, situated behind, and movably united with, the vertical limb of the prrcoperculum. There is an articulary cavity at its upper anterior angle for its junction with the hyomandibular. The interoperculum is connected by ligament with the angular piece of the mandible, and is also attached to the outer face of the hyoid, so that the gill-covers cannot open or shut without the hyoid apparatus executing a corresponding movement. The remaining membrane-bones are the urohyal (42), a single bone, which is connected by ligament with the anterior extremity of the humeral arch, and separates the musculi sternohyoidei, serving as an increased surface for their insertion ; and, finally, the branchiostegals (43), which vary greatly in number, but are always fixed to the ceratohyals and epihyals by ligaments ; the branchiostegal membrane is extended between them. 7. Dermal Bones of the Skull. To this category are referred some bones which are ossifications of, and belong to, the cutis. They are the turbinals (20), the suborbitals (19), and the supratemporals. They vary much with regard to the degree in which they are developed, and are seldom entirely absent. Nearly always they are wholly or partly transformed into tubes or hollows, in which the muciferous canals with their numerous nerves are lodged. Those in the temporal and scapulary regions are not always developed ; on the other hand, the series of those ossicles may be continued on to the trunk, accompanying the lateral line. The foremost suborbital is termed the prteorbital. The pectoral arch or shoulder-girdle of the Teleosteous Peetord fishes exhibits but a remnant of a primordial cartilage, arch - which is replaced by two ossifications, 1 the caracoid (51) and scapula (52) ; posteriorly they offer attachment to two series of short rods (53), of which the proximal are nearly always ossified, whilst the distal frequently remain small cartilaginous nodules hidden in the base of the pectoral rays ; they have been termed carpals and metacarpals. The bones by which this portion is connected with the skull are membrane-bones, viz., the clavicle (48), with the postclavicle (49 + 50), the supraclavicle (47), and the post-temporal (46). By this last bone the shoulder-girdle is suspended from the skull ; it is attached, in the perch, by a triple prong to the occipital and m.astoid bones. The clavicle completes the arch below by the symphysis or natural connexion of the bones of each side. Many Tele osteous fishes lack pectoral fins, and in them the pectoral arch is frequently more or less reduced or rudimentary, as in many species of Murcenidce. In others the membrane- bones are exceedingly strong, contributing to the outer protective armour of the fish, and then the clavicles are generally suturally connected in the median line. The postclavicula and the supraclavicula may be absent. It is only exceptionally that the shoulder-girdle is not suspended from the skull, but from the anterior portion of the spinous column (Symbranchidce, Murcenidce, Notacanthidct}. The number of elements in each of the two series of basalia never exceeds five, but may be smaller; arid the distal series is absent in Siluroids. The pubic bones (80) of the Teleosteous fishes undergo Pubic many modifications of form in the various families, but tones. they are essentially of the same simple type as in the perch, viz., a pair of flat or styliform simple bones, to which the ventral fins are articulated. The bones of the skull of the fish have received so many different interpretations that no two accounts agree in their nomenclature, so that their study is a matter of con siderable difficulty. The table of synonyms given on p. 644 will tend to overcome difficulties arising from this cause ; it contains the terms used for the different bones of the skeleton by Cuvier, those introduced by Owen, and finally the nomenclature of Stannius, Huxley, and Parker. Those adopted here are printed in italics. The numbers in the table are those used in the preceding pages and in figs. 24-28. Modifications of the Skeleton. We now proceed to pass briefly in review the modifi cations of the skeleton in the principal types of fishes, commencing at the lowest, and pointing out its gradual development in the other three subclasses. FIG. 29. Branchiostoma lanceolatum. a, mouth; c, vent; 6, abdominal porus. The lowermost subclass of fishes, which comprises one form only, the Lancelot (Branchiostoma [s. AmpJiiojcus~ lanceolalum), possesses the skeleton of the most primitive 1 Parker s nomenclature is adopted here.