An introduction to the study of fishes

by Albert C. L. G. Günther

Fig. 22.—Side and Front view of Fish-vertebra.

The vertebræ are either abdominal or caudal vertebræ, the coalescence of the parapophyses into a complete hæmal ring, and the suspension of the anal fin generally forming a sufficiently well-marked boundary between abdominal and caudal regions (Fig. [23]). In the Perch there are twenty-one abdominal and as many caudal vertebræ. The centrum of the first vertebra or atlas is very short, with the apophyses scarcely indicated, and lacking ribs like the succeeding vertebra. All the other abdominal vertebræ, with the exception of the last or two last, are provided with ribs, many of which are bifid (72). A series of flat spines (74), called interneurals, to which the spines and rays of the dorsal fins are articulated, are supported by the neural spines, the strength of the neurals and interneurals corresponding to that of the dermal spines (75). The caudal vertebræ differ from the abdominal in having the hæmapophyseal elements converted into spines similar to the neurals, the anterior being likewise destined to support a series of interhæmals (79), to which the anal rays are articulated. The last and smallest caudal vertebra articulates with the hypural (70), a fan-like bone, which, together with the dilated hindmost neural and hæmal elements, supports the caudal rays.

Looking at a perch’s skull from the side (Fig. [24]), the most superficial bones will be found to be those of the jaws, a chain of thin bones round the lower half of the eye, and the opercles.

The anterior margin of the upper jaw is formed by the intermaxillary or premaxillary (17) which bears teeth, terminates in a pedicle above, to allow of a forward sliding motion of the jaw, and is dilated into a flat triangular process behind, on which leans the second bone of the upper jaw, the maxillary (18). This bone is toothless, articulates with the vomer and palatine bone, and is greatly dilated towards its distal extremity. Both the maxillary and intermaxillary lie and move parallel to each other, being connected by a narrow membrane; in many other fishes their relative position is very different.

The mandible or lower jaw consists of a right and left ramus; their union by a ligament in front is called symphysis. Each ramus is formed of several pieces; that which, by a sigmoid concavity articulates with the quadrate, is the articulary bone (35); it sends upwards a coronoid process, to which a ligament from the maxillary and the masticatory muscles are attached; and forwards a long-pointed process, to be sheathed in the deep notch of the anterior piece. A small separate piece (36) at the lower posterior angle of the mandible is termed angular. The largest piece (34) is tooth-bearing, and hence termed dentary; at its inner surface it is always deeply excavated, to receive a cylindrical cartilage, called Meckel’s cartilage, the remains of an embryonic condition of the jaw, the articulary and angular being but ossified parts of it. In other Teleostei this number is still more increased by a splenial and other bones.

The infraorbital ring of bones (Fig. [23], ¹⁹) consists of several (four) pieces, of which the anterior is the largest, and distinguished as præorbital.

The so-called præoperculum (30) belongs rather to the bones of the suspensorium of the mandible, presently to be described, than to the opercles proper. It is narrow, strong, angularly bent, so as to consist of a vertical and horizontal limb, with an incompletely closed canal running along both limbs. As it is quite a superficial bone, and frequently armed with various spines, its form and configuration form an important item in the descriptive details of many fishes.

The principal piece of the gill-cover is the operculum (28), triangular in shape, situated behind, and movably united with, the vertical limb of the præoperculum. There is an articulary cavity at its upper anterior angle for its junction with the hyomandibular. The oblong lamella below the operculum is the sub-operculum (32), and the one in front of this latter, below the horizontal limb of the præoperculum, is the interoperculum (33), which is connected by ligament with the angular piece of the lower jaw, 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 chain of flat bones which, after the removal of the temporal muscles, appear arranged within the inner concavity of the præoperculum (Fig. [24]), are comprised with the latter under the common name of mandibulary suspensorium. They connect the mandible with the cranium. The uppermost, the epitympanic or hyomandibular (23), is articulated by a double articulary head with the mastoid and posterior frontal. Another articulary head is destined for the opercular joint. The mesotympanic or symplectic (31) appears as a styliform prolongation of the lower part of the hyomandibular; is entirely cartilaginous in the young, but nearly entirely ossified in the adult. The position of this bone is noteworthy, because, directly inwards of its cartilaginous junction with the hyomandibular, there is situated the uppermost piece of the hyoid arch, the stylohyal. The next bone of the series is the pretympanic or metapterygoid (27), a flat bone forming a bridge towards the pterygoid, and not rarely absent in the teleosteous sub-class. Finally, the large triangular hypo-tympanic or quadrate (26) has a large condyle for the mandibulary joint.

The palatine arch (Fig. [26]) connects the suspensorium with the anterior extremity of the skull, and is formed by three bones: the entopterygoid (25), an oblong and thin bone attached to the inner border of the palatine and pterygoid, and increasing the surface of the bony roof of the mouth towards the median line; it constitutes also the floor of the orbit. The pterygoid (24) (or os transversum) starts from the quadrate, and is joined by suture to the palatine, which is toothed, and reaches to the vomer and anterior frontal.