Section III—Pleuroperitoneal Cavity and Viscera

Open the pleuroperitoneal cavity by making an incision through the midventral body wall extending from about 1 cm posterior to the last pharyngeal slit to just anterior to the cloaca.

The most notable feature of the lamprey viscera, as with much of the lamprey’s anatomy, is its relative simplicity. This is often manifest in the apparent absence of struc­tures, as noted below. Keep in mind, however, that an absence or apparent lack of complexity in a particular structure does not necessarily indicate primitiveness.

On opening the cavity, the gonad may be the first struc­ture visible (Figures 2.4, 2.5). It is normally of moder-

FIGURE 2.5 Ventral view of pleuroperitoneal cavity of the lamprey, showing detail of gonad, kidney, and mesentery.

ate size but in breeding season it may be huge, occupy­ing much of the pleuroperitoneal cavity. The ovary and testis are, secondarily, single median structures, sup­ported by a mesentery from the middorsal line (Figure 2.5), and lying dorsal to the liver and intestine. The ovary and testis are difficult to distinguish from each other. Just before breeding, however, the ovary contains many follicles, giving it a granular appearance com­pared to the testis. Genital ducts are absent in the lamprey, and gametes are shed into the coelom. They must make their way to the posterior end of the pleu­roperitoneal cavity, where they exit by way of genital pores to enter the urogenital sinus (the single median cavity within the urogenital papilla). You may, after removing the gonad (see below) gently probe the poste­rior end of the pleuroperitoneal cavity, on either side of the urogenital sinus, to locate the genital pores; they are, however, difficult to find.

The liver (Figure 2.4) is the large, greenish organ at the anterior end of the pleuroperitoneal cavity. If the ovary is very large, it may cover the posterior end of the liver. A gall bladder and bile duct are present in the larval stage, but absent in adult lampreys. The digestive system is relatively simple, and consists mainly of a long, narrow, tubular intestine (though it may be distended in some specimens) extending the length of the pleu­roperitoneal cavity. There are none of the bends and folds so common in more derived vertebrates. The ante­rior part of the intestine extends from the “esophagus” (see below and Figure 2.8) and is dorsal to the liver, but more posteriorly it is easily seen in ventral view. Note the lack of any real regional specialization along the digestive tract. There is no stomach, for example. Given the mainly liquid diet of lampreys, there is no need for a separate stomach to temporarily store food and “feed” it to the intestine. Nor is there a distinct pylorus. A ter­minal swelling, sometimes present, may be recognized as a rectum. Make a longitudinal slit along a portion of

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FIGURE 2.6 Intestine of the lamprey, cut to show its internal structure.

the intestine and spread it open. Numerous longitudinal folds are present on its interior surface to increase surface area (Figure 2.6). One of these, the typhlosole, is notably larger than the others. It is also termed a spiral valve but its relationship to the spiral valve of other vertebrates is ambiguous. Also note the virtual absence of either dorsal or ventral mesenteries. A few small mesenteric sheets of dorsal mesentery, carrying blood vessels to the gut, are present near the posterior end of the intestine.

A spleen is absent, as is a distinct pancreas. Tissue per­forming pancreatic functions is present, however, and scattered through some viscera. Exocrine pancreas is present in parts of the intestinal wall and islet tissue (endocrine) occurs in the liver, but they are not visible grossly.

A long, thin, ribbon-like kidney lies on either side of the middorsal line and extends for much of the length of the pleuroperitoneal cavity (Figure 2.7). The archinephric duct, which drains the kidney, lies along its free lateral margin. Make a transverse cut through the kidney and examine the cut section to observe the duct. Posteriorly the archinephric ducts enter the urogenital sinus, but it is impractical to attempt tracing them.