Section II—External Anatomy
The external anatomy of the perch (Figure 4.3) is similar in several aspects to that of the dogfish shark. The body, which may be subdivided into head, trunk, and tail regions, is generally streamlined, not surprising in a swimming fish, and there are several fins.
The constricted region connecting the trunk and tail regions is the caudal peduncle. Several differences are immediately apparent, however. The skin, for example, has numerous scales. Also, there is only a single opening on each side of the body for the exit of water from the pharynx, and the positions of the paired fins are quite different.Examine the head (Figure 4.3). On each side it bears a large eye, lacking lids. Posterior to it, the preopercular region, containing the bones that help support the jaws, and the opercular region, containing the opercular bones covering the gills, are easily recognizable. The large mouth is terminal in the perch but may be slightly dorsal or ventral in other teleosts. Note that the maxilla, a bone of the upper jaw, is free posteriorly, embedded in a fold of skin, and lacks teeth (see also Figure 4.1). The premaxilla is recognizable also, at the anterior end of the upper jaw, and can slide back and forth, thus allowing the perch (and most other teleosts) to protrude its jaws. Teeth are present on the premaxilla, as well as on the lower jaw or mandible. A naris can be found anterior to each eye. On each side of the head there are two nostrils, one anterior and the other posterior, opening into the nasal cavities. Water enters the nasal cavity through the anterior nostril and exits through the posterior nostril.
On the trunk and caudal peduncle, the prominent lateral line forms a distinct ridge along the scales. Other canals occur on the head but they are much less conspicuous. Using forceps, pull out one of the scales from the trunk (Figure 4.4).
Most of the scale is embedded in the skin, and only a small posterior portion is exposed. This posterior end, termed the bony portion (though it is distinct from true bone), has numerous small tooth-like projections called ctenii. This type of scale is termed ctenoid (comb-like), based on the structure of its posterior end (other teleosts may have circular or cycloid scales because they lack ctenii; some lack scales). The embedded portion of the scale is made of fibrous connective tissue. Scales grow as the fish age. The concentric growth rings on the embedded portion of the scale can be used to age an individual.There are four median fins, the anterior dorsal fin, posterior dorsal fin, anal fin, and caudal fin (Figure 4.3). Identify the anterior and posterior dorsal fins along the dorsal midline. The anterior dorsal fin is larger and is supported by ossified fin rays, as noted above. Most of the supporting fin rays in the posterior dorsal fin are unossified and flexible. The anal fin is on the ventral midline, just anterior to the tail, and is supported mainly by soft fin rays. The homocercal caudal fin is superficially symmetrical. Note the paired fins, the pectoral and
FIGURE 4.3 External features of the perch in left lateral view.
FIGURE 4.4 Detail of scale of the perch in lateral view.
pelvic fins. Although some teleosts (for example, the catfish) have these fins in positions comparable to those in the shark, in the perch the pectoral fin is displaced dorsally and the pelvic is displaced anteriorly.
Lastly, examine the posterior openings of the urogenital and digestive tracts. Unlike the shark, the perch does not have a cloaca. Instead, the digestive tract has a separate opening, an anus, the large, circular opening anterior to the anal fin. The urogenital aperture is considerably smaller and less evident, and lies immediately posterior to the anus. In some females (see below), however, the urogenital opening may be as large as and even larger than the anus.
Key Terms: External Anatomy
anal fin
anterior dorsal fin anus
caudal fin
caudal peduncle ctenoid
eye
head
lateral line mandible maxilla
mouth
naris (nostril) pectoral fin pelvic fin posterior dorsal fin premaxilla
scales
teeth
trunk
urogenital aperture
FIGURE 4.5 Gills of the perch in left lateral view.