Perch Digestive System

            The digestive tract of the perch (a carnivore) includes the gut tube and the accessory digestive glands such as the liver and pancreas.  To expose the internal organs, hold the fish with the ventral side up and the head pointing away from you.  Insert the point of your scissors through the body wall in front of the anus and cut up the midline of the body to the space between the opercula (Figure 1).  Now lay the fish on its right side in the dissecting pan, and remove the skin and the muscles of its left side. To do that, continue to cut up around the back edge of the gill chamber to the top of the body cavity.  Make another incision from the starting point of the ventral incision close to the anus, and cut upward to the top of the body cavity (Figure 1).  Be careful not to disturb the internal organs.  Remove the lateral body wall by cutting along the top of the body cavity.  This procedure will expose the body organs in their normal position. 

Figure 1:  Incision lines for the dissection  of the perch.

Find the following internal organs:

a)  Esophagus - located at the extreme anterior end of the body cavity, the esophagus is a short, straight tube leading from the oropharynx to the stomach.

b)  Stomach -the stomach is a larger, thick-walled U-shaped tube.  In the perch breakdown of the food by mechanical and chemical means, begins in the stomach.  The size of the stomach varies according to how much food it contains.

c)  Pyloric caeca - the junction of the stomach and the intestine is marked by the presence of three pyloric caeca.  These are blind-ended tubes extending from the gut that serve secretory and absorptive functions.

d)  Intestine - originating at the stomach, the intestine forms an S-shaped loop.  At the end of the loop the intestine constricts and straightens.  The intestine extends directly to the anus.  The perch's intestine is less than the length of its body. This correlates with the animal's carnivorous life style.  Herbivorous fish have an intestine, which is longer (2 to 15 times the body length).  A longer intestine is required to provide greater digestive and absorptive surfaces for the herbivores.

e)  Liver -situated just anterior to the stomach. On the undersurface of the liver is the gall bladder.  The gall bladder drains bile from the liver, and opens by a number of ducts into the intestine.  Bile is necessary for the proper digestion of fats.

f)  Pancreas -the pancreas is a digestive gland, usually found along the ventral border of the intestine.  In some fish the pancreas is embedded in the liver.  The pancreas secretes digestive enzymes into the intestine and hormones (insulin and glucagons) into the blood.

g)  Spleen- lying on the posterior dorsal surface of the stomach, the spleen is a football shaped organ.  It functions in the production and maintenance of blood cells.

Figure 2:  Perch internal anatomy.