Download Fish Dissection

Fish dissection
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First observe the external anatomy of the perch. Feel the fish's skin. The slime protects against
growth of fungus and it helps fish slide through water. Look at the color pattern. Why is the
fish dark on top and light on the belly?
Your fish may or may not have spots. The fish do not have the same markings all its life
because they may change environments. Stream and ocean environments require different
protective coloration or camouflage. Observe the lateral line. What it is used for?
Why is the fish shaped the way it is?
How is this an advantage?
Notice the fish’s fins. The two dorsal fins, the pectoral fins, the pelvic fins, the anal fin (if it is
there), and the caudal fin. Look at placement of the fins and imagine a fish swimming in the
water. Note the range of movement of each fin. Pectorals can rotate 180 degrees; other fins are
less flexible. How does the fish move forward?
How are the other fins used?
Feel the boney rays that support the fins. Count the number of rays on the anal fin.
This is one of the distinguishing characteristics among different types of fish.
Note the size of the eye. Its relatively large size, and the large pupil, tells us how important
vision is for this animal. Notice that there are no eyelids. Observe the tough, clear membrane
that covers the eye. Rotate the eye in its socket with your finger. Take out an eye and see if you
can remove the lens that is used to focus light onto the retina, just like a human eye.
Locate the nostrils. They are blind pouches and do not connect to the mouth cavity. Their
sensory structures are attached to the brain. Why are their smell receptors so highly
developed?
Open the mouth. Are both jaws of the fish equally moveable? Explain.
Feel the teeth along the gum margins and on the roof of the mouth. Fish do not chew, but teeth
are used for grasping and holding prey. Find the rough, teeth-like structures on the tongue,
which help get food down. Does the tongue feel like a human tongue?
See how wide the mouth can open. Why is this?
The mouth is also used for breathing. In low oxygen conditions, fish can actively pump water
over their gills by opening and closing their mouth. Try this, relating it to the action of a pump.
The gills arches can be seen by looking down the fish's mouth. Use a probe to separate the
arches and explore how they are arranged. Place the fish on its side and look at the operculum,
the boney plates which protect the gills. Lift the operculum and look at the gills. Now cut the
operculum away at its base, exposing the gills. Remove the gills by cutting the upper and lower
attachments of the arch. Look at the gill rakers, the boney projections along the inside curve of
the arches. Observe the large surface area provided by the gill filaments, and the thin tissue
that allows blood vessels to come into contact with the oxygen in the water. What is one
difference between gills and lungs?
Locate the anus on the perch anterior to the anal fin. In the female, the anus is in front of the
genital pore, and the urinary pore is located behind the genital pore. The male has only one
pore – the urogenital pore – behind the anus. Determine the sex of your perch.
To expose the internal organs, hold the fish with the ventral side up and the head pointing away
from you. Carefully 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. Now lay the fish on its
right side in the dissecting pan. 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. 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.
Before moving any organs, see how everything fits together. Look for the thin, transparent
membrane that encloses the organs. Look for the swim bladder. It is made of very thin tissue
and is located in the upper body cavity, below the kidneys. It will be less well developed in small
fish and since it will not be inflated you may not see it. What is its function?
The male reproductive organs will be flaccid white or orange tissue near the intestines. Eggs
may or may not be noticeable in females. Both will vary in size depending on maturity of the
fish. Put the fish on its back and find the kidneys, located just under the backbone. They are
thin, dark in color, and run the whole length of the body cavity. What are the two functions of
the kidneys?
Locate the cream colored liver in the front of the body cavity, and the gall bladder that rests
between the lobes of the liver. A fish's heart is almost literally in its mouth. It can be found by
removing the liver. You should be able to make out the different chambers. The heart pumps
blood to the gills and liver first. Pumping blood to the gills makes sense. What does the liver
do that makes it just as important to the fish?
Investigate the digestive track by starting in the mouth and following the route that food would
take. Put the probe through the mouth and into the esophagus to show the beginning of the
route. Then follow the course of the stomach using your finger or the probe. The first area of
the stomach is where digestion begins. It is thick and U-shaped. You may want to cut it open
to see what the fish has been eating. In between the stomach and the coiled intestine are the
pyloric ceca, which aid in digestion by secreting digestive enzymes and helping to absorb
nutrients. Notice the network of blood vessels in the intestine. Why are they there?
Follow the intestines to the anal opening where waste products are eliminated. 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 that is longer (2 to 15 times the body length). A longer
intestine is required to provide greater digestive and absorptive surfaces for the herbivores.
The spleen is posterior and dorsal to the stomach. It is a football shaped reddish organ. It
functions in the production and maintenance of blood cells, destroying those that are old and
recycling the hemoglobin. The pancreas and liver are accessory digestive glands. The pancreas
is usually found along the ventral border of the intestine. In some fish it is embedded in the
liver. What does the pancreas do?
Cut through the fish to expose the backbone and muscles. Observe arrangement of the muscle
masses. This is the part of the fish we eat. Carefully cut away the skin by lifting it while running
the scalpel along the skin-muscle interface. Use a magnifying glass to observe the pattern of the
scales, the growth rings on the scales, and the darkly pigmented spots. Draw a scale.
How many growth rings are there on your scale?
Why do fish have scales?