2. Membrane-bones attached to the primordial skull.—To this group belong the parietals (7) and frontals (1). The squamosal (12) has been mentioned above in connection with the mastoid. The supraorbital is always small, and frequently absent. The lower surface of the skull is protected by the basisphenoid (parasphenoid) (6) and the vomer (16), both of which, especially the latter, may be armed with teeth.[9]
3. Cartilage bones of the alimentary portion of the visceral skeleton of the skull.—The suspensorium consists of three cartilage-bones, and affords a base for the opercular apparatus as well as a point of attachment to the hyoid, whilst in front it is connected with the palato-pterygo-palatine arch. They are the hyomandibular (23), symplectic (31), and quadrate (26), connected by means of the metapterygoid (27) with the ecto- (24) and ento-pterygoid (25), the foremost bone of the arch being the palatine (22). All these bones have been sufficiently described above (p. 55), and it remains only to be mentioned that the bones of the palatine arch are but rarely absent, as for instance in Murænophis; and that the symplectic does not extend to the articulary of the mandible, as in Amia and Lepidosteus, though its suspensory relation to the Meckelian cartilage is still indicated by a ligament which connects the two pieces. Of the mandibulary bones the articulary (35) is distinctly part of Meckel’s cartilage. Frequently another portion of cartilage below the articulary remains persistent, or is replaced by a separate membrane-bone, the angular.
4. Membrane-bones of the alimentary portion of the visceral skeleton of the skull.—The suspensorium has one tegumentary bone attached to it, viz. the præoperculum (30); it is but rarely absent, for instance in Murænophis. The premaxillary (17) and maxillary (18) of the Teleostei appear to be also membrane-bones, although they are clearly analogous to the upper labial cartilages of the Sharks. The premaxillaries sometimes coalesce into a single piece (as in Diodon, Mormyrus), or they are firmly united with the maxillaries (as in all Gymnodonts, Serrasalmo, etc.) The relative position and connection of these two bones differs much, and is a valuable character in the discrimination of the various families. In some, the front margin of the jaw is formed by the premaxillary only, the two bones having a parallel position, as it has been described in the Perch (p. 53); in others, the premaxillary is shortened, allowing the maxillary to enter, and to complete, the margin of the upper jaw; and finally, in many no part of the maxillary is situated behind the premaxillary, but the entire bone is attached to the end of the premaxillary, forming its continuation. In the last case the maxillary may be quite abortive. The mobility of the upper jaw is greatest in those fishes in which the premaxillary alone forms its margin. The form of the premaxillary is subject to great variation: the beak of Belone, Xiphias is formed by the prolonged and coalesced premaxillaries. The maxillary consists sometimes of one piece, sometimes of two or three. The principal membrane-bone of the mandible is the dentary (34), to which is added the angular (36) and rarely a smaller one, the splenial or os operculare, which is situated at the inside of the articulary.
5. Cartilage-bones of the respiratory portion of the visceral skeleton of the skull.—With few exceptions all the ossifications of the hyoid and branchial arches, as described above (p. 58), belong to this group.
6. Membrane-bones of the respiratory portion of the visceral skeleton of the skull.—They are the following: the opercular pieces, viz. operculum (28), sub-operculum (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 urohyal (42) which separates the musculi sternohyoidei, and serves for an increased surface of their insertion; and finally the branchiostegals (43), which vary greatly in number, but are always fixed to the cerato- and epi-hyals.
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 rarely 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. In many fishes those of the infraorbital ring are much dilated, protecting the entire space between the orbit and the rim of the præoperculum; in others, especially those which have the angle of the præoperculum armed with a powerful spine, the infraorbital ring emits a process towards the spine, which thus serves as a stay or support of this weapon (Scorpænidæ, Cottidæ).
The pectoral arch of the Teleosteous fishes exhibits but a remnant of a primordial cartilage, which is replaced by two ossifications,[10] the coracoid (51) and scapula (52); they offer posteriorly attachment to two series of short rods, of which the proximal are nearly always ossified, whilst the distal frequently remain small cartilaginous nodules hidden in the base of the pectoral rays. The bones, by which this portion is connected with the skull, are membrane-bones, viz. the clavicle (49), with the postclavicle (49 + 50), the supraclavicle (47), and post-temporal (46). The order of their arrangement in the Perch has been described above (p. 59). However, many Teleosteous fish lack pectoral fins, and in them the pectoral arch is frequently more or less reduced or rudimentary, as in many species of Murænidæ. 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. Only exceptionally the shoulder-girdle is not suspended from the skull, but from the anterior portion of the spinous column (Symbranchidæ, Murænidæ, Notacanthidæ). The number of basal elements of each of the two series never exceeds five, but may be less; and the distal series is absent in Siluroids.
The pubic bones of the Teleosteous fishes undergo many modifications of form in the various families, but they are essentially of the same simple type as in the Perch.
CHAPTER V.
MYOLOGY.
In the lowest vertebrate, Branchiostoma, the whole of the muscular mass is arranged in a longitudinal band running along each side of the body; it is vertically divided into a number of flakes or segments (myocommas) by aponeurotic septa, which serve as the surfaces of insertion to the muscular fibres. But this muscular band has no connection with the notochord except in its foremost portion, where some relation has been formed to the visceral skeleton. A very thin muscular layer covers the abdomen.