Section III—Mouth, Oral Cavity, and Pharynx

Examine the mouth, which forms the anterior opening of the digestive tract. It is terminal, a position common in fish that swim to overtake their prey. Note the pos­terior end of the maxilla.

Expose the oral cavity and pharynx by removing the opecular bones as follows. Lift the free, posterior end of the operculum. Insert one blade of a stout pair of scis­sors beneath the surface of the operculum, at approxi­mately the midheight of the posterior margin. Keeping the blade close to the deep surface of the operculum, cut through the bones, heading toward and through the angle of the mouth. Spread the flaps to observe the gills, each composed of numerous gill filaments, which are involved with respiration (or gas exchange) with the water flowing over them. Then cut away, bit by bit, the opercular flaps covering the gill until you have exposed the region, as shown in Figure 4.5. The most anterior of the four branchial arches, and the gill it supports, should be plainly visible. Manipulate the arches and gills to identify the remaining three arches. Gill rakers should be plainly visible on the first arch. The rakers are projections that extend inward across the pharyngeal slit. They help in feeding, preventing prey (and other debris) from passing through the pharyngeal openings and escaping.

In respiration, water passes through the mouth and oral cavity into the pharynx. It passes over the gills and into the opercular chamber (which corresponds to the parabranchial chambers seen in the shark), the space between the gills and operculum, and then leaves pos­teriorly through the opening at the posterior end of the operculum. The floor of the opercular chamber is sup­ported by the branchiostegal rays.

There are five passages or slits through the pharynx: Three are between the four brachial arches, one is ante­rior to the first arch, and another is posterior to the last arch. Each arch bears a double set of filaments. This is similar to the condition in the holobranch of the shark, in which gill lamellae are present on the anterior and posterior surfaces of the interbranchial septum (see page 46). In the holobranch of the perch, however, the septum is absent, so the gill filaments are positioned almost side by side. Still, they correspond to the ante­rior and posterior sets of lamellae of the shark, with the anterior filaments being posttrematic (“after the slit”) and the posterior being pretrematic (“before the slit”). The perch thus has four holobranchs. Unlike in the shark, there is no hemibranch (and, of course, no pseudobranch, as the spiracle is absent).

Key Terms: Mouth, Oral Cavity, and Pharynx