Download Perch Dissection Guide

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.