Section I—Skeleton

Skull

Examine a mounted skeleton of the perch and differen­tiate the head, trunk, and tail regions (Figure 4.1). Unlike the Chondrichthyes, the Actinopterygii have well-developed bony skeletons.

Begin by locating the opercular bones on the pharyn­geal region that cover the gills (Figures 4.1 and 4.2). The operculum is the large, triangular bony plate. Several bones surrounding the operculum can be easily identi­fied. The L-shaped preoperculum lies anterior to the operculum; the suboperculum posteroventral to the operculum. Ventral to the preoperculum is the inter­operculum. The four branchial arches that support the gills lie deep to the operculuar bones. Peer beneath the operculars to observe the arches.

Next, locate the large, circular orbit that houses the eyeball. Some of the deeper bones of the skull, such as the parasphenoid, are visible within the orbit. A series of bones surround and help form the orbit. Several are large and easily identifiable, such as the frontal, which

FIGURE 4.1 Skeleton of the perch in left lateral view.

FIGURE 4.2 Skull and branchial skeleton of the perch in left lateral view.

forms much of the skull roof, and the lacrimal, which forms the anteroventral margin of the orbit.

A series of three main bones, lying between the orbit and preoperculum, extend anteroventrally. These are, in dorsal to ventral order, the hyomandibular, metaptery­goid, and quadrate. These bones contribute to the sus­pensorium, the apparatus that supports the jaws on the rest of the skull. The hyomandibular and metapterygoid support the quadrate, which forms the articulation with the lower jaw. The lower jaw is formed by three bones. The articular forms most of the posterior end of the lower jaw and articulates with the quadrate of the upper jaw. The dentary, which bears teeth, may be seen ante­rior to it. The angular is a small bone posteroventrally on the lower jaw.

Key Terms: Skull

Postcranial Skeleton

The vertebral column includes trunk vertebrae anteri­orly and caudal vertebrae posteriorly (Figure 4.1). The main part of a vertebra is the centrum.

Examine the anterior and posterior dorsal fins on the middorsal line (Figure 4.1). The fins are supported by thin, elongated fin rays. These rays may be ossified, as are those supporting the anterior dorsal fin, and usually termed spines. Other fin rays, termed soft fin rays, are flexible and unossified and may branch distally. Each fin ray is supported at its base by a pterygiophore, a ven­trally tapered bony element. The tip of a pterygiophore extends ventrally into the connective tissue between two neural spines. Note that the pterygiophore series is continuous—they extend all the way along the dorsum beneath and between the anterior and posterior dorsal fins. The separation between these fins, in fact, is due to reduction of the spines between them. The posterior dorsal fin is structured similarly to the anterior dorsal fin, except that only the first two fin rays are spines, the remainder being soft fin rays.

The anal fin lies along the midventral line, opposite the position of the posterior dorsal fin. As in the latter, all but the first two fin rays are soft rays. Pterygiophores support the fin rays, essentially as in the dorsal fins, but taper dorsally. The anterior few (usually two) pterygio- phores fuse into a large element that extends dorsally to attach to one or two ventral ribs, which here are quite reduced in size. It is posterior to this point of attach­ment that the series of hemal spines begins.

The caudal fin is supported by soft fin rays, which are arranged to form a homocercal tail. This type of fin is superficially symmetrical with about equal numbers of fin rays dorsal and ventral to the longitudinal axis extending posteriorly from the vertebral column. However, the body axis itself turns abruptly dorsally, so it is not symmetrical internally, although this is not easy to detect with the naked eye (see below). This upward turn is easily noted in less derived teleosts, such as stur­geons. It may still be appreciated in advanced forms such as the perch by the orientation of the uroneurals, the last few neural spines, and the position of the hypu- rals immediately posterior to them. The hypurals, also present ventral to the uroneurals, are the flattened hemal spines of the last few caudal vertebrae and provide most of the support for the fin rays of the caudal fin. Several neural spines, termed epurals and unattached to verte­brae, provide some support for the dorsal part of the fin.

Lastly, examine the paired fins (Figures 4.1 and 4.2). The cleithrum is the main supporting element of the pec­toral girdle. It is a large, dorsoventrally elongated bone that lies mainly deep to the operculum and subopercu­lum, and extends to the ventral midline to articulate with the cleithrum from the other side of the body. Dor- sally it articulates with the supracleithrum, which, in turn, articulates with the posttemporal. The latter is attached to the posterior end of the skull. This chain of connections links the head and shoulder girdle. The scapula and procoracoid are the large skeletal elements of the shoulder girdle that directly support the fin. The scapula lies dorsal to the procoracoid. Their anterior ends lie deep to and are covered by the cleithrum. The postcleithrum is an elongated triangular bone that extends dorsoventrally. Its widened end lies deep to the cleithrium. It tapers ventrally, passes medial to the fin, and extends toward the pelvic fin. The radials, of which there are usually four, extend from the scapula or pro­coracoid and distally articulate with the soft fin rays supporting the fin.

The pelvic girdle is formed by paired basipterygia. Each is a triangular plate of bone oriented anteroposteriorly and with base located posteriorly. As it passes anteri­orly, the basipterygium tapers and passes dorsal to the articulation between the right and left cleithra. The fin rays of each fin attach directly to the posterior end of a basipterygium, and, except for the medial ray, are soft fin rays.

Key Terms: Postcranial Skeleton

procoracoid pterygiophore radials scapula

supracleithrum trunk vertebrae uroneurals ventral ribs