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«Digestive Systems: The Anatomy of Representative Vertebrates Modified from: Biology in the laboratory. 3rd edition. Helms, Helms, Kosinski and ...»

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Digestive Systems: The Anatomy of Representative Vertebrates

Modified from:

Biology in the laboratory. 3rd edition. Helms, Helms, Kosinski and Cummings.

Biological Investigations: Form, Function, Diversity and Process. 7th Edition. W.D. Dolphin

Helms, Helms, Kosinski, Cummings. Biology in the Laboratory, 3rd edition. Freeman Publishing.

Harold M. Kaplan and Kathleen A. Jones Southern Illinois University

OVERVIEW

The digestive system participates in the procurement and metabolism of energy-containing materials.

Food is taken in through the mouth and digested in the digestive tract, and nutrients are transported to all parts

of the body by the circulatory system. Molecules obtained as food are stored at a variety of sites in the body:

glycogen is stored in the liver and muscle tissues; fats are stored in the liver and muscles in basal vertebrates and in adipose tissues in more advanced vertebrates. The components of proteins (amino acids), however, are not stored-they are used to build new proteins or are de-aminated in the liver to make simple sugars or fatty acids, which can be converted to storage molecules.

Digestion involves the chemical breakdown of complex food materials through the actions of enzymes.

Chemically, it requires hydrolysis of the covalent bonds holding together large, polymeric molecules. Proteins are hydrolyzed into amino acids and starches into sugars. The smaller molecules resulting from digestion are easily absorbed. These small molecules are used by the animal either as sources of energy or as building blocks to make new molecules, such as proteins and nucleic acids that are characteristic for the species. Thus, when we digest bovine proteins in a hamburger, we reuse the amino acids to make human proteins or convert the amino acids to sugars that are used as an energy source.

Digestion can occur intracellularly or extracellularly. In simple organisms, such as protozoa and sponges, the individual cells of the organism ingest food materials by pinocytosis and phagocytosis and digestion occurs in food vacuoles inside of cells. Other organisms have extracellular digestion in special digestive organs that are either sac-like or tubular. Multicellular organisms, such as cnidarians and flatworms, have incomplete digestive tracts; these are blind sacs; that is, food enters the mouth, passes into a chamber where enzymatic digestion and absorption occur, and nondigestible material is expelled through the same opening. This seems to be a somewhat inefficient system because, when nondigestible material is expelled, recently ingested food may also be lost.

More complex animals have a complete digestive system, a continuous tube from the mouth to the anus in which food is sequentially broken down and absorbed. Different enzymes are secreted by glands at various points along the digestive tube, so that the digestion of different types of molecules occurs as food passes through the system.

Mere digestion, however, is not sufficient for processing food material. To be of value to the animal, absorption must also occur. In vertebrates, specialized regions of the digestive tube absorb nutrients, salts, and water from the digested material, or chyme. The undigested or nondigestible residues pass on to temporary storage areas before being defecated.

Arthropods and annelids have a digestive tube that passes more or less straight through the body from head to tail. The digestive tubes in such organisms include storage areas, grinding areas, digestive areas, and absorptive areas, but the tube is not longer than the organism. In vertebrates, on the other hand, the digestive tube is many times longer than the animal's length, with much of this length devoted to absorption. After absorption, the circulatory system carries food materials to cells throughout the body.

–  –  –

Learning Objectives:

• Describe the differences between incomplete and complete digestive tracts

• Outline the pathway of food though mammalian digestive tract

• Describe the anatomy of Hydra gastrovascular cavity

• Compare and contrast the anatomy of organs of the Vertebrate digestive system and explain how they’re anatomy reveals their function.

As a result of this review, you most likely have questions about terms, concepts, or how you will do the experiments included in this lab. Write these questions in your lab notebook.

Digestion Lab - Must have in your lab notebook Diagrams

1. Gastrovascular cavity of a Hydra, Cnidarian OR Ctenophore a. Comparing anatomy of one-opening and two-opening digestive tubes Dissections

2. Frog - Labeled a. Divisions of the body cavity b. Digestive organs in mesentaries and in body cavity c. Food remains – ID or color and consistency (if no remains, they should mention that; otherwise ½ point off)

3. Pig – labeled a. Digestive organs in the body c avity

–  –  –

2 Invertebrate Feeding Behavior in Hydra Hydra is a small, freshwater Cnidarian related to the jellyfish and sea anemones. It lives attached to submerged rocks, leaves, and twigs. Hydra's body is organized simply, consisting of only two layers of cells surrounding a hollow cavity. However, the organism is highly specialized for food gathering; it uses tentacles to capture food and transfer it into the digestive (gastrovascular) cavity. The food is then digested by enzymes that are secreted by cells lining the cavity. Because of the organism's small size, the cells of Hydra obtain nutrients from the digestive cavity by simple diffusion. Each body cell exchanges O2 and CO2 directly with the surrounding water and releases cellular metabolic wastes directly into the water as well.





Recall how Hydra feeds:

Include drawings showing how prey items are transferred from the tentacles to mouth.

Once food is in Hydra's gastrovascular cavity, digestion begins. Enzymes secreted by certain cells lining the cavities begin extracellular digestion. Partially digested bits of food material are later taken up by phagocytic cells in the cavity lining, and further digestion occurs inside the food vacuoles in these cells. This is intracellular digestion. Food absorbed by the cells lining the gastrovascular cavity supplies all cells of the body.

When animals increase in size and complexity, diffusion from the digestive cavity to the cells, as occurs in Hydra, is no longer adequate to supply the tissues' demands. Increased complexity means special systems for the transport of material from cell to cell. The relationship of the vertebrate digestive and circulatory systems represents the height of this development from physiological and evolutionary perspectives.

Ceolom and Mesenteries The coelom is a body cavity lined by mesoderm. In many invertebrates, a body cavity filled with fluid (hydrocoel) functions as a supporting skeleton for the body. However, in vertebrates the endoskeleton assumes this role, and the major role of the coelom in the course of evolution has been to allow the internal organs to lengthen, coil, and move independently of the outer body wall.

Over the course of Vertebrate evolution, the coelom (originally a single cavity) was subdivided into several body cavities containing different organs (Figure 1a). Early on, the heart came to occupy the pericardial cavity and was separated from the abdominal cavity by the transverse septum. As swim bladders or lungs developed in Vertebrates, they began to protrude into the abdominal cavity (pleuroperitoneal cavity) at the level of the heart and behind it (Figure 1b). As the development of the neck separated the head from the body, a lateral fold of the body wall joined the transverse septum, forming a pleuropericardial membrane separating the heart from the lungs (Figure 1c). In advanced vertebrates, the pleural cavities encasing the lungs became completely separated from the abdominal cavity by the pleuroperitoneal membranes (and transverse septum), which in mammals form the diaphragm, a muscular structure assisting the muscles of the body wall in ventilating the lungs (Figure 1d).

Coelomic body cavities are lined with a thin, permeable epithelium. On the inside surface of the body wall, this lining is called the parietal peritoneum. Visceral organs in the body cavity are suspended by extensions of the peritoneum called mesenteries (Table 1). Mesenteries contain small amounts of connective tissue, blood vessels, and nerves, and in advanced vertebrates may also contain adipose (fat) tissue. Body organs are covered by the visceral peritoneum, also part of the living of the coelom.

The heart tends to be free of suspensory mesenteries or ligaments which could constrain its activity. The pericardial cavity containing the heart is lined by a parietal pericardium and the heart is covered by the visceral pericardium.

–  –  –

4 Objectives

• Open the coelomic cavities of representative vertebrates and describe how the coelom of each is subdivided.

• Distinguish among the parietal peritoneum, the visceral peritoneum, and the parietal and visceral pericardia and be able to locate each.

• Describe the structure and the major functions of the mesenteries.

Procedure It is customary to give dissection directions (right and left) in terms of the animal's right and left. When the animal's ventral surface is toward you, the animal's right is on your left and vice versa. We will follow this convention. Work in groups of two.

Frog

1. Turn your frog ventral side up. Using scissors make a shallow longitudinal cut through the thin skin of the ventral surface of the body (the skin will separate easily from the underlying muscles). Extend the cut from just in front of the cloacal aperture to the pectoral girdle. Make transverse cuts along the ventral surface of the forelimbs and hindlimbs as if making a large letter I. Free the skin from the underlying trunk and limbs and pull it back.

2. Now cut through the muscle layers of the ventral body wall slightly to the right of the midventral line (your left as you face the frog). Extend the cut from in front of the cloacal aperture to the hind margin of the forelimbs (you may need to veer left as you approach the pectoral girdle to avoid the sternum). Be careful not to damage the visceral organs and the heart. Lift up the left-hand flap of the body wall and locate the large ventral abdominal vein on its inner surface along the midline (this is why you cut to one side of the median plane). Carefully separate the vein from the inner surface of the body wall (use a probe or dissecting needle). Make transverse incisions through the body wall anterior to each)1 hind leg and posterior to each front limb (avoid the vein). Extend these cuts about halfway up the side of the body. Rinse the preservative and dried blood from the coelomic cavity. Pour any preservative that has collected in your dissecting tray into the sink.

3. Pull the flaps of the body wall outward. If your dissecting pan has a wax lining, use dissecting pins to hold these flaps away from the opening into the coelomic cavities. The large pleuroperitoneal cavity houses the lungs and abdominal viscera. With the head pointed away from you, push the viscera to the frog's right to see the nature of the peritoneal wall and mesenteries.

• Describe the appearance of the parietal peritoneum

• Describe the appearance of the mesenteries.

• What other organs or tissues are contained within them in the frog?

4. Find the transverse septum that forms the anterior wall of the pleuroperitoneal cavity. The liver, a large dark organ filling much of the anterior pleuroperitoneal cavity, is attached to it by the coronary ligament.

5. Find the heart, nestled between the lobes of the liver behind the pectoral girdle. It is located in the pericardial cavity.

• What structure separates the pericardial cavity from the pleuroperitoneal cavity?

• Does this separation appear to be complete in the frog?

5 The Digestive System In primitive vertebrates, the digestive system consisted of little more than an anterior opening, the mouth; the pharynx; a foregut and a hindgut separated by a constriction (the pylorus); and a posterior opening, the anus.

The first vertebrates were filter-feeders, feeding continuously on small particles of food suspended in the water.

With the development of jaws, larger food items were taken at less regular intervals ("meals"), and a temporary storage area, the stomach, developed in the anterior part of the system. The remainder of the digestive system changed little during the course of evolution. See Figure 2.

Oral Cavity: The oral cavity was formed as jaws evolved to enclose a chamber between the anterior opening of the digestive tract (mouth) and the pharynx.

Pharynx: Behind the oral cavity is the pharynx. Its primary role is associated with gas exchange, so we will postpone our exploration of this area.

Foregut: The foregut extends from the pharynx to the pyloric constriction and often expands near the pylorus to form a storage organ, the stomach. The anterior portion of the foregut forms a connecting tube, the esophagus.

–  –  –

Figure 2: The complete gut. A generalized schematic diagram of structures and organs associated with the digestive tract in various vertebrates. Note that no single vertebrate possesses all of these structures.

When jaws developed, they enclosed a space in front of the pharynx, the oral cavity. Teeth lined the jaws and, with the change from gill to pulmonary respiration, the pharyngeal arches were modified to support a tongue used to gather and manipulate food. In terrestrial vertebrates, teeth became embedded in the bone of the jaw, anterior flaps of skin (lips) formed, and the angle of the jaw became closed (forming cheeks). In the evolutionary line leading to lobe-finned fishes and terrestrial vertebrates, internal nares arose and connected the roof of the oral cavity to the outside through nasal passages. In terrestrial vertebrates, salivary glands evolved to provide lubricating moisture for dry food and to protect the epithelial lining of the digestive tract.

The oral cavity evolved into the most complicated part of the digestive system, adapting to the wide range of diets in vertebrates.



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