Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Perch Dissection By Howard Hagerman External anatomy of the perch Select a specimen and place it left surface up in a dissection pan. Before studying the gross external anatomy, remove a scale from the body and examine it under l00 × of the microscope. What type of scale is it? Can you determine the number of growing seasons that the specimen has lived? Begin studying the external anatomy by first noting the general regions of the fish. The head extends from the tip of the snout to the posterior tip of the operculum (gill cover). There is no neck and thus the head runs directly into the trunk region which terminates at the anus located about two-thirds the length of the specimen on the ventral surface. The tail begins behind the anus and terminates at the last vertebra. The distance measured from the anterior tip to the last vertebra is called the standard length of the fish. The caudal fin, which is not included in standard length, is inserted in the flesh behind the last vertebra and is included in the total length. For taxonomic purposes, average standard lengths are usually given whereas the sports fisherman wanting to boost his "bragging rights" will invariably use total length in describing the catch. The trunk is divided into several areas for descriptive purposes. Most useful taxonomically are the positions of the fins. Just posterior to the operculum are the pectoral fins, which are homologous to the shoulder and fore limb-region of other vertebrates. The pelvic fins of the perch are only slightly posterior and ventral to the pectorals. The entire, less pigmented ventral area is usually called the belly (not very scientific, but descriptive none-the-less). The dorsal fin is composed of two distinct sections; the anterior most section has spiny rays and posterior section has soft rays. This two-parted dorsal fin is a family characteristic of the Percidae. Just posterior to the anus on the ventral surface of the animal is the anal fin with one sharp spiny ray and the remaining ones soft. The number of ray~ in selected fins as well as their position on the body are often used as key characteristics for genus and species separations. The homocercal caudal fin is the propulsion mechanism of the animal. The thin lateral line located just dorsal to the mid-lateral area of the body extends from the operculum to the caudal fin. It serves sensory functions for the animal. It might be instructive for you to microscopically compare a scale from the lateral line with one taken from some other part of the body. Study the head region of the fish. The area between the anterior orbit of the eye and front of the animal is called the snout; it has a pair of openings, the nostrils located about half way back on the dorsal part of the snout. The structures of the upper and lower jaws may be of taxonomic value. The forward, upper lip is called the premaxilla and just posterior and slightly ventral to that is the maxilla. The lower jaw is called the mandible and of course, as in other jawed vertebrates, is hinged. We have already mentioned the operculum or gill covering, which is also divided into several parts. The eye is of special interest. Can you find a lid or membrane covering it? Why would such a membrane or lid not be necessary in this animal? At what point in phylogeny would one expect such an eye covering? Internal anatomy With sharp scissors remove the operculum by cutting to just behind the orbit. Be careful to cut only the operculum and not the underlying gills. How many pairs of gills does the animal possess? Remove the posterior gill by clipping its connectives and lifting it out with forceps. The filaments are soft structures well supplied with blood and serve as areas of gas exchange. These are supported by bony structures (cartilage in some), the gill arches. How many arches are there? How many gill arches would one find in a primitive chordate (e.g., the lamprey)? Where are the missing ones? With the scissors, cut a large section out of the body beginning at the posterior edge of the last gill and extending dorsal to the pectoral fin, through the ribs and arching toward the anus. Cut around the anus and then back forward between the pelvic fins to the posterior edge of the former operculum. Lift out this large section of flesh and bone and view the underlying structures. First study the digestive tract and its associated organs. The false diaphragm separates the abdominal cavity posterior from the anterior pericardial cavity that is located between and ventral to the gills. The first abdominal organ that you should see is the brownish liver. Gently pull the liver back from the diaphragm and note two veins that run from the liver through the diaphragm to the heart. The gall bladder is attached to the right lobe of the liver. Behind the liver is the sac-like stomach. It is peculiar in that the esophagus from the pharynx and the intestine from the stomach enter and leave respectively at the same end of the stomach. Such a close-ended stomach is called a caecal stomach. The intestine loops back over itself once as it extends toward the anus. Close to the stomach the intestine bears several blind pouches called pyloric caecae. The organs of the digestive tract are held in place by membranes which connect organ to organ as well as organ to body wall. These are called mesenteries. These are extensions of the linings of the body cavity. They therefore, function in defining the coelom of the fish. Incorporated in the mesentery dorsal to the intestine is the darkened spleen. What is its function? Cut the intestine about one centimeter forward of the anus and begin gently lifting it out of the body cavity. The dorsal mesentery will likely inhibit this process and will therefore need to be cut away as the intestine is lifted forward. Care should be taken to not remove any structures but the intestinal tract and the spleen. The tract may be cut just forward of the stomach and completely cleared away. The air (swim) bladder may be located as a tough membranous structure closely applied to the dorsal surface of the body cavity. In life the air bladder would be inflated, but it will likely be collapsed in this specimen. Does the air bladder have a homologue in other vertebrates? What is its function in this organism? To help answer that question, the darters (which are in the same family) lack an air bladder. These fish live in streams and lakes and are bottom dwellers. The air bladder opens into the pharynx through tiny ducts that are not likely to be seen with the naked eye. Placing the specimen beneath the dissecting microscope may be helpful in locating these ducts. Dorsal to the air bladder is the dark colored kidney. It may be necessary for you to depress the air bladder in order to observe the kidney. What type of kidney is this? Since the digestive tract has been removed and we have detached the air bladder, the remaining structures in the body cavity are associated with reproduction. It is not possible for us to determine male from female until we have opened the cavity. In the male a pair of long, milk white testes lie just dorsal to where the intestine had been. They had to be moved out of the way in order to completely study the intestine. The two testes are united along the posterior one third of their median surfaces and terminate as a single duct called the vas deferens. Sperms are discharged through this duct to the outside via the urogenital opening, which is common with the urinary discharge as well. In the female there is a single two-parted ovary lying in the middle of the abdominal cavity. The ovary is within a membranous ovarial sac that is attached to the ventral body wall at its posterior end, between the anal and urinary openings. Unlike most vertebrate organisms there is no permanent opening for discharge of eggs, but instead when the eggs are mature, they are discharged through a temporary rupture between the urinary opening and the anus. Since the eggs are discharged in water where currents may separate them, a gelatinous matrix that is secreted by subsidiary ovarian tissue conveniently holds them together. Now turn your attention to the area forward of the abdominal cavity, the pericardial cavity. Study as many structures of circulation as you can on the specimen. An injected specimen has been dissected to help you in locating some of the parts. The heart consists of two chambers, ventricle and auricle. The two chambers may be differentiated by position and texture. The ventricle is ventral and anterior and is thick walled while the auricle is thin walled posterior to the ventricle and slightly dorsal. Thus the heart is tilted in the cavity with the auricle dorsal and the ventricle ventral. The auricle receives blood from the body through veins which coverage just before the auricle into a single enlargement called the sinus venosus. On the ventricle side of the heart it should be possible to observe the bulbous arteriosus. Extending forward from the bulbous is the ventral aorta that branches four times - one for each pair of gills; these branches are termed afferent arteries. From here on the blood vessels of the un-injected specimen may be very difficult to see and therefore, I would urge you to look at the demonstration specimen. Blood from the afferent arteries passes into the gill filaments where gaseous exchange occurs. As the blood is surged into the vessels of the gills it enters into tiny capillary beds where exchange surface is greatly increased. In addition to the increase in surface, the blood flows in the opposite direction of the water over the gills. This counter current phenomenon is responsible for a powerful exchange force that allows the animal to extract up to 85% of the oxygen dissolved in the water passing over the gill. Oxygenated blood leaves the gill via the efferent branchial arteries that pass dorsally to the dorsal aorta. Blood in the aorta passes anteriorly to bathe the head region and posteriorly to supply blood to the visceral organs and to be cleansed in the kidneys. Many tiny branches also carry blood to the muscles. The fish is of course typical of animals with closed circulation in which blood leaves the heart through arteries - enters into capillaries - and returns to the heart through veins. Since the fish is only a two-chambered heart, blood goes to the gills and out to the body before returning to the heart. How does this compare with a mammal? Remove a 1.5 x 2 inch patch of scales and skin from the tail region of the specimen. This will expose the body musculature that gives form to the animal as well as serving in locomotion. The muscles form a large lazy W as they are viewed from dorsal to ventral. Each muscle segment is called a myotome and functions by rhythmically contracting from anterior to posterior alternately right and left side so as to cause the animal to undulate in waves that have an accumulative effect toward the tail. The thrust of the tail against the resistance of the water propels the animal forward. Study of the nervous system will be the final exercise of this lab. Cut into the bones of the cranium with sharp scissors by inserting the tip of the scissors into the external nares and cutting posteriorly. Our intention here is to expose the brain by cutting and chipping away bones of the cranial cavity. Once the cavity has been exposed, carefully remove the fat cells that cover the brain. Careful work should expose the major divisions of the brain. Beginning anteriorly the olfactory nerves are paired and pass posteriorly to the olfactory bulb. What is olfaction? Behind the olfactory bulb is an enlargement of the brain called the telencephalon. What mammalian brain structure would it have as a homologue? Next is a bilobed brain structure that on its ventral side receives the impulses of sight; these are called the optic lobes (tecta). Notice their size compared with the rest of the brain. It is obvious the emphasis that the fish places on sight and smell from the amount of nervous tissue set aside for these sensory functions. The cerebellum lies posterior to the optic lobes. Following this brain structure, the medulla oblongata completes the brain and runs imperceptibly into the spinal cord. A darkened membrane called the choroids plexus covers the dorsal surface of the medulla. This membrane partially conceals an opening into the brain stem; the opening is the fourth ventricle. Although we will not try to locate them, the fish possesses ten cranial nerves (some authorities say eleven) which receive sensory signals and pass them to the brain for clearing, interpretation and action as is needed for the organism's well being (or perceived well being). Place the dissected specimen and all scraps in the designated container. Scrape out and rinse out the dissecting pan. Clean up any other mess.