Section IV—Digestive and Respiratory Systems

Anteriorly the digestive system includes the mouth, oral cavity, and pharynx, but these structures also function in the respiratory system. To observe them, make a frontal cut, using scissors, through the gills on the side of the head that was skinned for the muscles.

FIGURE 3.22 Anterior portion of the shark in ventral view. The right side visceral arches have been cut through to reflect the floor of the oral cavity and pharynx. The esophagus and stomach have also been cut and reflected.

The mouth is the opening into the oral cavity. The oral cavity is bounded anteriorly by the teeth. Its floor is formed by the primary tongue, which is not a true tongue as occurs in tetrapods. The boundary between the oral cavity and pharynx is not clearly defined in the adult. The pharynx is considered the region into which the pharyngeal slits lead. Posteriorly it narrows into the esophagus, which leads to the stomach. Observe the five internal pharyngeal slits. Gill rakers are the finger-like structures projecting across the slit that help keep food from escaping through the slits or damaging the gills, which are the organs of respiration (i.e., across which gas exchange occurs). The spiracle, a smaller and dor- sally placed opening, lies in front of the first pharyngeal slit. The spiracle, along with the mouth, allows water to enter the pharynx, from which it passes out through the pharyngeal slits. The spiracle is larger and more important as a passageway for water in bottom­dwelling skates and rays.

Examine the branchial region along the cut you made to open the mouth. Note the four interbranchial septa, the partitions that separate the five pharyngeal slits. The anterior and posterior surfaces of each septum bear a gill (Figures 3.20, 3.22), which is made of parallel, ridge-like primary gill lamellae. A septum that has lamellae on both anterior and posterior surfaces is termed a holobranch. A hemibranch has lamellae on only one side, as occurs with the first gill, in which lamellae are present on the posterior wall. Keep in mind that holobranch and hemibranch are used with refer­ence to lamellae or septa and not to pharyngeal slits.

FIGURE 3.23 Branchial region of the shark in dorsolateral view, with blowup showing detail of pretrematic and posttrematic surfaces of successive interbranchial septa.

Reexamine a septum in section (Figures 3.20 and 3.22). It consists of a medial cartilaginous support, the branchial arch. Review the cartilages of the arch if nec­essary. A branchial adductor muscle lies medial to the arch. Extending laterally is a muscular wall, the inter- branchial muscle, which supports lamellae on its ante­rior and posterior surfaces. Cartilaginous gill rays (see Figure 3.20) extend from the arch into and help support the interbranchial muscle. Further laterally, the septum is completed by the superficial constrictor muscle. A pre- trematic artery lies at the base of the lamellae on the posterior surface of the septum, and a posttrematic artery lies anteriorly. These names are used with refer­ence to a pharyngeal slit, trema being the ancient Greek word for hole or slit. Probe the septum for cross trunks linking these arteries. An afferent branchial artery lies near the middle of the septum between the pretrematic and posttrematic arteries.

The celom or body cavity includes the pericardial cavity anteriorly and the pleuroperitoneal cavity posteriorly.

To gain access to this cavity, make a longitudinal cut, slightly to one side of the midline, from the posterior side of the pectoral girdle to the base of the tail. In doing so, cut through the puboischiadic bar and to one side of the cloaca. Then make two transverse cuts on either side to produce four flaps as shown in Figure 3.24.

Note that you have cut through various layers to open the pleuroperitoneal cavity: skin, musculature, and parietal peritoneum, the epithelium that lines the body cavity (Figure 3.24). The epithelium covering the organs within the cavity is the visceral peritoneum. The organs are suspended from the body wall or connected to each other via membranes termed mesenteries. Any mem­brane in the peritoneal cavity that supports an organ may be called a mesentery. One mesentery is simply called the mesentery, and you will meet that below.

First, examine the viscera. The most conspicuous is the liver, which occupies most of the anterior part of the

FIGURE 3.24 Pleuroperitoneal cavity of the shark in ventral view, showing viscera and vessels.

cavity. Right and left lobes extend posteriorly on either side. Do not cut off these lobes (otherwise, you will have to contend with a continuous leakage of the oil present in the liver that reduces the shark’s specific gravity). A small, median lobe extends for a short distance between them and contains an elongated gall bladder.

Spread the lobes of the liver to expose the viscera completely. Entering the cavity dorsal to the liver is the esophagus. It passes posteriorly into the stomach, a large J-shaped organ. There is no external distinction between the stomach and esophagus.

The main part of the stomach is the body. The smaller, narrower, posterior part is the pyloric region. It con­stricts at the pylorus, which marks the separation between the stomach and intestine. Two organs lie near the junction of the stomach and intestine. The triangu­lar spleen is the large, dark-colored organ at the poste­rior end of the stomach. It is not part of the digestive system, but belongs to the cardiovascular system. The pancreas consists of two parts linked by a narrow isthmus. A flattened, oval ventral lobe (Figure 3.24) lies on the anteroventral surface of the intestine. The narrow, elongated dorsal lobe extends posteriorly.

The intestine of the shark is subdivided into a short duo­denum, a valvular intestine, and a narrow colon. The valvular intestine bears a spiral valve, an internal sub­division that increases the effective length of the intes­tine. Slit open the intestine to see the valve (Figure 3.25). The colon, lacking a spiral valve, extends from the valvular intestine. It is joined by the salt-excreting dig- itiform gland, before continuing into the cloaca as the rectum.

Other structures in the pleuroperitoneal cavity should be noted only briefly at this time. The gonads, either paired testes or ovaries, lie anteriorly, dorsal to the liver (see Figures 3.33 and 3.34). The kidneys extend longi­tudinally along the dorsal wall of the cavity on either side of the sagittal plane as two narrow strips. These organs, as well as various ducts associated with the uro­genital system, will be considered later.

Next, consider the mesenteries. Pull the digestive tract ventrally and to the right. Note the dorsal mesentery, the thin, translucent sheet that suspends the gut mid- dorsally (Figure 3.26).

FIGURE 3.25 Cutaway view of the valvular intestine and pos­terior end of stomach of the shark in ventral view, revealing the structure of the spiral valve.

tery can refer to any membrane that supports viscera) supports the anterior part of the intestine. Gently probe the mesentery and note that it arises from the meso- gaster. Pull the digitiform gland ventrally to observe the mesorectum. Finally, gently pull the spleen posteriorly and note that it is attached to the stomach via the gastrosplenic ligament.

In the embryo a complete ventral mesentery is also present, but only portions of this membrane remain in the adult. Spread the median and right lobes of the liver. A ribbon-like strand, the gastrohepatoduodenal liga­ment (or lesser omentum) will be seen extending from the liver toward the gut. Near the liver it is a single bundle, carrying the bile duct and supporting blood vessels between the liver and gut. As it approaches the pyloric region, it divides into a hepatoduodenal liga­ment that carries the bile duct to the duodenum, and a hepatogastric ligament that passes into the adjacent por­tions of the body and pyloric region of the stomach. Look between these ligaments and note that the pan­creas is supported by a part of the dorsal mesentery. The falciform ligament extends between anteroventral surface of the liver and the mid-ventral body wall. This

FIGURE 3.26 Pleuroperitoneal cavity of the shark in right lateral view, with intestine pulled ventrally and liver reflected to expose mesenteries.

short mesentery also supports the common openings of the oviducts, described later.

Key Terms: Digestive and

afferent branchial artery bile duct

body of stomach

branchial adductor muscle

celom

colon

cross trunks between pretrematic and posttrematic arteries digitiform gland (rectal gland)

Respiratory Systems

duodenum

esophagus falciform ligament gall bladder gastrohepatoduodenal ligament (lesser omentum) gastrosplenic ligament

(lienogastric ligament) gill rakers gill rays

gills gonads hemibranch hepatoduodenal

ligament hepatogastric ligament holobranch interbranchial muscle interbranchial septa isthmus of pancreas kidneys left lobe of liver liver median lobe of liver mesenteries—general

sense

mesentery—specific sense

mesogaster (greater omentum)

mesorectum ovaries pancreas, dorsal lobe pancreas, ventral lobe papillae (sing., papilla) parietal peritoneum pericardial cavity pleuroperitoneal cavity posttrematic artery pretrematic artery primary gill lamellae primary tongue