Section IV—Pleuroperitoneal Cavity and Viscera
To expose the pleuroperitoneal cavity and viscera, make an incision along the midventral line, proceeding as follows. In a female with an enlarged urogenital opening, insert a scissor blade into the anterolateral margin of the aperture.
Make a small incision, approximately 0.5 cm, in this direction, and then turn back anteromedially toward the midline, anterior to the anus. This will avoid injuring the digestive tract. In a specimen with a small urogenital aperture, proceed as just described, but begin by inserting the scissor blade into the anus. Continue to cut anteriorly along the midline, keeping close to the deep surface of the body wall, past the pelvic girdle. Turn your incision dorsally to pass posterior to the attachment of the pectoral fin on the body. Cut dorsally past the pectoral fin approximately to the level posterior to the eye.Return to the posterior end of the midventral incision. Cut dorsally for about 1 cm, then reflect the flap of body wall and examine the dorsal part of the cavity. You should see a dark, membranous sac, the swim bladder. Probe it gently. Resume cutting through the body wall until you reach the level just ventral to the swim bladder (this point will be ventral to the position of the dor- salmost point of the anterior vertical incision). Then cut anteriorly, more or less parallel to the swim bladder, to join the anterior vertical incision. Your incision should veer slightly dorsally as you cut. In addition to the musculature of the body wall, you will also cut through the ventral ribs. Once finished, remove the section of body wall and examine the underlying structures. The viscera are covered by visceral peritoneum, whereas the cavity itself is lined by parietal peritoneum.
Many female specimens will possess a very large ovary that seems to fill most of the pleuroperitoneal cavity (Figure 4.6).
In such specimens you will probably only be able to see the darkly colored liver at the very anterior end of the cavity. Find the narrow small intestine ventral to the ovary. It is normally midventral, but may be displaced to one side by a massive ovary. Note the swim bladder dorsal to the ovary. In specimens with smaller gonads, several other visceral structures may be observed without further dissection. The small intestine is easily identifiable on the floor of the cavity. Posterior to the liver, you should observe the short, thick stomach, and perhaps the spleen and gall bladder if you have opened the right side of the cavity.Once you have identified these structures, remove more of the body wall, preferably in small pieces, as follows. Remove the wall anterior to the liver, keeping close to the deep surface of the wall and continuously checking that you are not destroying underlying structures. Immediately anterior to the liver is the transverse septum, a thin membrane that separates the pleuroperitoneal cavity and the pericardial cavity. Cut through the transverse septum along its attachment to the body wall to expose the heart within the pericardial cavity.
At this stage, continue the midventral incision anteriorly to the posterior margin of the mouth, as shown in Figures 4.8 and 4.9. The musculature is thicker here, so cut carefully to avoid damaging the heart. You may wish to use a scalpel. Also cut away the lateral portions of the branchial arches to expose the pharynx. Then remove more of the lateral body wall dorsal to the liver. This will expose a small, dark, lobulated mass, the head
FIGURE 4.6 Left lateral view of the perch with body wall cut away to reveal pharynx and pleuroperitoneal cavity.
kidney, lying immediately dorsal to the liver. Once you have exposed the head kidney, use a scalpel to cut a parasagittal section through the musculature dorsal to the swim bladder.
This will allow you to expose the kidneys, which lie against the dorsal wall of the cavity, dorsal to the swim bladder. Be careful in using the scalpel. It is worth removing a row or two of scales along the path you intend to cut. If this method proves too awkward, find the kidneys by removing the swim bladder, but do so after you have examined the remaining structures described below.The preparation described above is time-consuming, but it reveals the pattern, context, and arrangement of the various systems and their structures in a single view. Although you can begin with any of the structures, it is best to examine the gonads first, because in many specimens they will be so large that they will have to be removed.
The ovary of the female will vary considerably in size with the reproductive cycle of the fish, and may be massive, filled with eggs (Figure 4.6). The ovary of the perch is secondarily fused into a single structure (although this is not true of most teleosts, which retain paired ovaries) that is enveloped during embryonic development by bilateral peritoneal folds. This envelopment continues posteriorly and meets a funnel-like internal elongation of the urogenital aperture, lying just posterior to the anus. This combination (i.e., of the peritoneum and internal elongation of the aperture) forms an ovarian duct for passage of the eggs, although it is not comparable to that (usually considered a Mullerian duct) of other vertebrates. Gently tugging the posterior end of the ovary will pull taut the funnel-shaped posterior end of the ovarian duct and make it easier to distinguish. In other teleosts the ovarian duct is formed differently. For example, a common pattern is that the ovary contains an internal cavity, formed by envelopment of a small part of the celomic cavity during embryonic development. The eggs are shed into this cavity, the lining of which extends posteriorly to form an ovarian duct. In most teleosts, therefore, the eggs are released directly into a tube, the ovarian duct, rather than into the pleuroperitoneal cavity, as occurs in almost all other vertebrates.
Cut transversely through the ovary, approximately 3 cm from its posterior margin, and carefully remove the anterior portion. This will leave a cone-shaped posterior end in place. Gently reflect it ventrally and delicately dissect between the ovary and swim bladder, now clearly visible, to expose the small, light-colored, elongated, and oval urinary bladder (Figure 4.7). The bladder continues posteroventrally into the urinary opening of the urogenital pore, but postpone tracing it.
The paired testes of the male perch are lobulated, lightcolored, and posteriorly tapered structures (Figure 4.8). Each testis has its own duct, the testicular duct, that carries only sperm. The testicular duct is a specialization of teleosts (although not all teleosts, such as salmonids, possess a duct for the testis; instead, sperm are released by the testes into the body cavity and leave the body through pores) and is not comparable to the archinephric duct observed in sharks. It is fairly small and thus difficult to find without a dissecting microscope. Right and left testicular ducts unite near the posterior end of the testes into a single duct that leads out of the body through the urogenital pore just posterior to the anus. The opening of the duct can be distinguished from that of the archinephric duct (see below) with a magnifying glass. The urinary bladder lies dorsal to the posterior end of the testes.
FIGURE 4.7 Schematic illustration showing the urogenital structures of the female perch in right lateral view.
Turn your attention to the anterior end of the animal (Figures 4.8 and 4.9). Identify the oral cavity—look now for the various teeth described above—and follow it posteriorly into the pharynx. The pharynx leads into the wide, short, and straight esophagus that passes posteriorly into the stomach. The stomach is “T”-shaped, with a broad horizontal portion and a short, vertical, pyloric portion forming the stem of the “T”.
The coiled intestine follows the pyloric portion. Note the three finger-like projections, the pyloric ceca, at the anterior end of the intestine. These are typically present in teleosts, though their number varies. The anterior part of the intestine, the duodenum, is somewhat wider than the remaining distal portion. Although its terminal portion may be referred to as a rectum, it is not sharply demarcated from the rest of the intestine.Note various structures associated with the digestive tract. The large, massive liver has already been noted. It may be necessary to remove part of it, as shown in Figures 4.8 and 4.9, in order for the stomach and other organs to be seen properly. The gall bladder is a small, elongated sac. The bile duct leads to the duodenum, but
FIGURE 4.8 Cutaway view of the male perch in right lateral view, to reveal structures of the pharynx and pleuroperitoneal cavity.
FIGURE 4.9 Cutaway view of the female perch in left lateral view, to reveal structures of the pharynx and pleuroperitoneal cavity. Ovary has been removed.
it is difficult to find. The pancreas cannot be seen grossly. Bits of pancreatic tissue are scattered throughout the mesentery, often embedded in the fatty tissue there (Figure 4.9). The spleen, not properly an organ of the digestive system but concerned with production of blood cells, is an elongated, dark-colored structure near the posterior end of the stomach.
The swim bladder is the large, hollow sac lying, as noted above, dorsally in the body cavity. It is not enclosed by the peritoneum, and so is retroperitoneal in position. It is a hydrostatic organ used to control buoyancy. Its inflation decreases the fish’s density, thus increasing buoyancy. Its deflation has the opposite effect. The bladder develops as an outgrowth of the anterior part of the digestive tract, and in many teleosts it retains an open duct connection to the esophagus, a condition termed physostomus.
In the perch, however, the connection between the bladder and gut is lost, the physo- clistous condition.The kidneys lie dorsal to the swim bladder and are thus also retroperitoneal. They are long, narrow, ribbon-like structures with somewhat scalloped lateral margins lying on either side of the dorsal midline of the body cavity. Posteriorly, the kidneys curve ventrally, following the surface of the body cavity (Figure 4.7). Each kidney is drained by an archinephric duct (in the males of some species it may also receive sperm, but the more common condition is that represented by the perch, in which a separate testicular duct serves for sperm passage). The right and left ducts enter the urinary bladder. Urine exits the body through a single duct leading to the urinary opening of the urogenital aperture. Dissection of this region to reveal the ducts is difficult without a microscope and considerable patience. Figure 4.7 indicates the structures and their relationships.
Finally, examine the heart, which has already been exposed and noted. As in the shark, the heart is an S- shaped, four-chambered structure that receives venous blood posteriorly and pumps it anteriorly into the gills (Figures 4.8 and 4.9). The most posterior chamber is the sinus venosus, which directs blood into the atrium lying immediately anterior to it. From the atrium, blood enters the ventricle, which lies ventrally. The ventricle pumps blood through the fourth chamber, the bulbus arteriosus, which leads into the ventral aorta. Afferent branchial arteries branch off the ventral aorta, leading blood through the gills. Efferent branchial arteries recollect the blood into the dorsal aorta, which distributes it to the various parts of the body. Unless you have an injected specimen, the vessels will be difficult to follow.
Key Terms: Pleuroperitoneal Cavity and Viscera
archinephric duct
(Wolffian duct) atrium bulbus arteriosus duodenum esophagus gall bladder
head kidney heart kidneys liver ovarian duct ovary
pancreas
parietal peritoneum
pyloric cecum (plur., ceca)
rectum
sinus venosus
small intestine
spleen
stomach swim bladder testes
testicular duct transverse septum urinary bladder ventricle visceral peritoneum