Activity 2: Digestive System
Click on any photograph for an enlarged view in a separate window.
Click HERE to access the Activity 2 Dissection Booklet
Click HERE to view the Shark Dissection Video Part 2
Examine the photographs of the spiny dogfish shark with its body cavity slit open by clicking the blue lettered links in the column to the right. The shark specimen in the photographs was prepared by turning it ventral side up and making a mid-ventral incision just anterior to the cloacal opening. It was cut in an anterior direction slightly to the right of the mid-ventral line. This cut was continued all the way to the pectoral girdle. The inside of the large body cavity was exposed. The large flaps of body wall were folded back and pinned.
A smooth, shiny membrane called peritoneum can be seen lining the inside of the body wall. The visceral organs are suspended dorsally by a double membrane of peritoneum know as mesentery.
The liver is the largest organ Iying within the body cavity. Its two main lobes, the right and left lobes, extend from the pectoral girdle posteriorly most of the length of the cavity. A third lobe (the median lobe) a much shorter lobe is located medially and contains the green gall bladder along its right edge. The gall bladder's function is to store the bile which is produced by the liver.
The liver has three main functions:
1. it is an energy storage area for the shark since all fatty reserves are stored in this organ
2. it is also a hydrostatic organ which means that oils that are less dense than water are stored here, thus giving the shark some buoyancy
3. it is the main organ for the production of bile, which is used to help the shark digest fats and oils from the food it eats
Examine the photographs of the shark with its liver moved aside to show its stomach by clicking the blue lettered links in the column to the right.The shark specimen in the photographs was prepared by moving the large liver to the shark's right side.
The esophagus is the thick muscular tube extending from the top of the cavity connecting the oral cavity and pharynx with the stomach.
The esophagus leads into the "J"-shaped stomach. The upper portion, the cardiac region, continues as the main body, and ends at the duodenal end.
Examine the photographs of the shark with its stomach slit open by clicking the blue lettered links in the column to the right. The shark specimen in the photographs was prepared by cutting the stomach open along its long axis. This shark's stomach contained the partially digested remains of fish which were washed out under running water. The main function of the shark stomach is to serve as an organ of pre-digestion. The food is mixed here with digestive enzymes and churned by the strong muscles until it is a semi-liquid mixture.
The mucosa is the inner lining of the stomach. The rugae are longitudinal folds that help in the churning and mixing the food with digestive juices. The rugae also allow the stomach to expand to accommodate large meals. A circular muscular valve, the pyloric sphincter, is located at the far end or pyloric end of the stomach. It regulates the passage of partially digested food into the intestines.
Examine the photographs of shark with its liver moved aside to show its intestines by clicking the blue lettered links in the column to the right.The specimen in the photographs was prepared by moving the large liver forward.
The duodenum is a short "U"-shaped portion of the small intestine that connects the stomach to the intestine. The bile duct from the gall bladder enters the duodenum.
The pancreas is located on the duodenum and the lower stomach. The pancreas is involved in the production of hormones as well as digestive enzymes to help break down fats and carbs in the stomach. The secretions of the pancreas enter the duodenum by way of the pancreatic duct.
The dark, triangular-shaped spleen is located near the posterior end of the stomach. Although a part the Iymphatic system, the spleen is closely associated with the digestive organs in all vertebrates. The spleen is also involved in the production of red blood cells as well as being part of the shark's immune system.
The valvular intestine is the second, and much larger, portion of the small intestine. It follows the duodenum and its outer surface is marked by rings.
Examine the photographs of the shark with its valvular intestine slit open by clicking the blue lettered links in the column to the right.The shark specimen in the photographs was prepared by cutting away the outer tissue of the valvular intestine.
The spiral valve is the screw-like, symmetrical shape within the valvular intestine. It adds surface area for digestion and absorption to an otherwise relatively short intestine.
Examine the photographs of of the shark with its valvular intestine moved aside to show its cloaca by clicking the blue lettered links in the column to the right.The shark specimen in the photographs was prepared by pulling the intestine forward.
The colon is the narrowed continuation of the valvular intestine. It is located at the posterior end of the body cavity.
The rectal gland is a slender, blind-ended, finger-like structure that leads into the colon by means of a duct. It has been shown to excrete salt (NaCI) in concentrations higher than that of the shark's body fluids or sea water. It is thus an organ of osmoregulation, regulating the shark's salt balance.
The cloaca is the last portion of the alimentary canal. It collects the products of the colon as well as the urogenital ducts. It is a catch-all basin leading to the outside by means of the cloacal opening.
Not officially part of the digestive system, the kidneys can be seen along either side of the spinal column in the far back (or deep) section of the abdominal cavity. The kidneys are actually behind the abdominal cavity within their own membranous sacs. This is referred to as being retroperitoneal...meaning that the kidneys are behind the peritoneal cavity. The kidneys are involved with the filtering of the blood by removing wastes and forming urine. This is essentially the only function of the kidneys in the female shark. In the male shark, however, the anterior portion of the kidney (called the epididymis) is involved in the reproductive process.