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Biology 1001
Lab 10
FISH DISSECTION
PREPARATION
Read this exercise before coming to laboratory.
You will need a dissecting kit for this exercise.
TEXTBOOK READINGS
Campbell et al. (2008). Biology. Chapt. 34. Figure 34.16 (Anatomy of a trout).
OBJECTIVES
At the end of this laboratory you should be able to:
1. Explain the objectives of dissection and the importance of good dissection techniques.
2. Distinguish between the anatomical terms - anterior and posterior; dorsal and ventral;
caudal and cranial; transverse plane, frontal plane, saggital plane.
3. Locate and name the main external features of a bony fish.
4. Locate the gills, show the main parts of a gill, and explain the function of each part.
5. Identify the parts of the digestive system.
6. Describe the heart and locate its main parts.
7. Describe the basic pattern of circulation in a bony fish.
LABORATORY ASSIGNMENTS
1. Complete and label a drawing of the external anatomy of the yellow perch.
2. Complete and label a diagram of the internal anatomy of a yellow perch.
3. Answer the questions on the laboratory assignment pages.
INTRODUCTION TO DISSECTING
The chief objective in the dissection of a specimen is to expose body parts for the study of their
structure and their relationship to other parts. It is important to proceed carefully, following the
instructions completely.
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1. Your specimen, although now dead, was once a living animal. We are fortunate to have the bodies
of these animals to learn from. Treat your specimen with respect, do not betray its death. It is all too
easy to make a mess of dissection, which makes learning difficult and upsets your laboratory partners.
A good dissection should reveal all of the organs clearly, so that the essential relationships of the organs
can be seen. Anything less than this is pointless mutilation.
2. Dissecting tools are exceptionally sharp. Always cut gently, away from your fingers and body.
3. “Dissect” means “expose to view”. Dissecting consists of carefully separating structures and picking
away surrounding connective tissue in order to expose them clearly. Use the scalpel as little as possible
and only on tissue that requires cutting. In fact, few tissues besides the skin and certain muscles will
require cutting. A blunt probe or fine forceps is usually sufficient to tease apart tissues as you identify
body organs.
4. Do not cut out any organs unless specifically instructed to do so.
5. The terms right and left always refer to the specimen's right and left. Depending on how the
specimen is oriented, this may or may not correspond with your right and left.
6. You should protect your hands from the preservative (usually a mixture containing formalin) in the
specimen's tissues by wearing surgical gloves.
7. At the end of a laboratory always clean up your work area and put your specimen away in the
orange “Biohazard” bag provided. Wash out the dissecting pan and leave to drain. Wash and fully
dry your dissection instruments. If you choose to discard your scalpel blade, please use the “sharps
disposal” containers provided.
GLOSSARY OF KEY TERMS
Aboral - away from or opposite to the mouth.
Anterior - toward the front or head end; the opposite of posterior.
Asymmetry - an irregular arrangement of body parts; without a central point, axis, or plane of
symmetry.
Bilateral symmetry - an arrangement of body parts on opposite sides of a central plane (midsagittal
plane), which divides the body into two symmetrical halves (mirror images).
Caudal - toward the tail or tail end; the opposite of cephalic.
Cephalic - of or pertaining to the head; the opposite of caudal.
Cranial - relating to the skull or cranium.
Cross section - sections of the body cut on any transverse plane; such sections are perpendicular to
the sagittal and frontal planes.
Deep - pertaining to structures away from the surface of the body; the opposite of superficial.
Distal - away from the center or point of attachment; the opposite of proximal.
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Dorsal - relating to the back or upper surface; the opposite of ventral.
Frontal plane - plane parallel to the dorsal and ventral surfaces of the body, which bisects a bilaterally
symmetrical animal into upper and lower halves.
Lateral - toward the side; the opposite of medial.
Longitudinal - lengthwise; parallel to the long axis.
Medial - toward the sagittal plane or center of the body; the opposite of lateral.
Median - located in or near the sagittal plane.
Oral - relating to the mouth.
Peripheral - toward the outer surface.
Posterior - the hind part (rear) of the body; the opposite of anterior.
Proximal - toward the center or point of attachment; the opposite of distal.
Radial symmetry - arrangement of body parts symmetrically around a central axis; any plane through
the central axis divides the body into symmetrical halves (mirror images).
Sagittal plane - any longitudinal plane passing from the head to tail. The midsagittal plane bisects a
bilateral animal into two symmetrical halves (mirror images).
Superficial - located near the surface of the body; the opposite of deep.
Transverse plane - any plane perpendicular to the sagittal and frontal planes. Sections of the body
cut on a transverse plane are called cross sections.
Ventral - relating to the belly or underside; the opposite of dorsal.
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----------------------------------------------------TERMINOLOGY
Obtain a specimen and put it in a dissecting pan. Orient yourself to the anatomical directions of
the specimen by referring to the Glossary and Figure 10.1.
The back is the dorsal surface, while the belly is the ventral surface. The head is the cranial or
anterior end; the tail is the caudal or posterior end.
The three anatomical planes are shown in Figure 10.1. The transverse plane cuts through the
body at right angles to the back bone. It is like a cross-section and divides the body into cranial and
caudal parts. The frontal plane is a horizontal longitudinal section that divides the body into dorsal
and ventral parts. The saggital plane cuts through the body vertically from front to back and divides
it into left and right halves.
Make sure you understand the other terms in the Glossary as they are used to give you directions in
the dissecting instructions.
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I.
EXTERNAL STRUCTURE OF THE YELLOW PERCH
The yellow perch, Perca flavescens, is a common bony fish found in freshwater lakes and streams of
North America. It is native to the central United States and southern Canada, but is widely stocked
elsewhere. A similar species, Perca fluviatilis, the European perch, is common in Europe. Both perches
are representative of modern bony fishes.
The yellow perch is a member of the clade (or super-class) Osteichthyes, the bony fishes, the
largest clade of living vertebrates. Over 20,000 species of bony fishes have been described from fresh
and salt water habitats. Among the principal distinguishing features of the Osteichthyes are a bony
skeleton, terminal mouth, homocercal tail, dermal scales, paired nostrils and ears with three
semicircular canals. The yellow perch falls under the Class Actinopterygii, the ray-finned fishes. In
these fish, the fins (supported mainly by bony rays) are modified for maneuvering, defense and other
functions.
Obtain a preserved perch and put it in a dissecting pan. Cover the bottom of the pan with water to
keep the fish moist. Study the locations of the following structures.
The body of the perch is fusiform, or torpedo-shaped and is thickened about one-third of the
distance from the mouth to the tail and tapers in both directions. Pick up the fish and look directly at
the mouth from the front; observe the oval cross section of the fish. How would this shape facilitate
movement through the water?
Identify the three regions of the body - head, trunk and tail. The head extends to the posterior
edge of the operculum which covers the gills; the trunk extends from the operculum to the anus; the
tail extends from the anus posteriorly.
Identify the pectoral, pelvic, anal, dorsal, and caudal fins. How many of each are there?
Which are paired? Which are single? The fins are important for movement of the fish; they help in
stability and in directing the movements of the fish through the water. Note the fin rays, which
support the thin membrane of each fin. Are some of the rays soft and some spiny? Where are the spiny
rays located? The caudal fin is homocercal, meaning that the upper and lower halves are alike. The
caudal fins like those of the dogfish shark, which are asymmetric, are called heterocercal.
Locate the mouth, nostrils, eyes and lateral lines on your specimen. The lateral lines which
extend from the operculum to the base of the tail on both sides are one of the principal sensory systems
of the fish. Lateral lines are specialized sense organs that detect vibrations and current directions in the
water. They appear to function in orientation, in helping the fish avoid obstacles, and in escaping
predators.
Lift an operculum, the hard plate that covers the gills, and study its structure. Along the ventral
margin of the operculum, find a membrane supported by bony rays. This membrane fits snugly against
the body to close the branchial cavity during certain respiratory movements. With your probe,
examine the gills beneath the operculum.
Find the anus near the base of the anal fin and the small slit like urogenital opening just
posterior to the anus.
Note the arrangement of the scales. The scales, which are embedded in the tough skin, protect the
surface of the body and are arranged in a well-ordered pattern of longitudinal and diagonal rows. Note
how the posterior portion of one scale overlaps the anterior portion of the next one. The scales grow
continuously during the life of the fish and are not regenerated if lost. Seasonal variations in the growth
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of the fish are reflected in the rings on the scales and can be examined microscopically to determine the
age of the fish.
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LABORATORY ASSIGNMENT 1
On your laboratory assignment pages is an outline of a fish. Use this as the basis for making a
complete drawing of the external features of a yellow perch. Label all the terms that appear in bold in
the description.
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II. INTERNAL ANATOMY OF THE YELLOW PERCH.
Skeletal System
Look at the mounted perch skeletons on demonstration and at the drawing of the perch skeleton in
Figure 10. 2. Locate the axial skeleton, consisting of the bones of the skull, the vertebral column,
the ribs, and the medial fins. The appendicular skeleton is made up of the pectoral girdle, the
pectoral fins and the small pelvic girdle and fins.
A vertebrate skeleton has 3 main functions:
1. It supports the body.
2. It protects delicate structures.
3. It forms a site for the attachment of muscles.
In addition, it also serves as:
1. Levers for muscular action.
2. A site for the production of blood cells (in the bone marrow).
3. A reserve of stored minerals.
------------------------------------------------------DEMONSTRATION:
On demonstration today are skeletons of other members of the subphylum Vertebrata. There are
representatives from the following classes:
1.
2.
3.
4.
5.
Super-class Osteichthyes - the bony fishes
Class Amphibia - the amphibians
Class Reptilia - the reptiles
Class Aves - the birds
Class Mammalia - the mammals
Please examine these skeletons and note the similarities between them. The basic arrangement of
bones remains the same, particularly among the terrestrial animals. Differences in these animals such as
size, shape, posture and type of locomotion is achieved by changing the relative length and size of
individual bones in the skeleton. This is sometimes accompanied by the fusion of two or more bones,
or the reduction or loss of bones. Figure 10.3 shows the human skeleton and the names of some of its
bones.
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Muscular system.
Though the muscles of the perch are less complex than those of land vertebrates, they make up a much
larger mass in relation to body size. Tetrapod locomotion results largely from direct action of muscles
on bones of the limbs, but fish locomotion results from the indirect action of the segmental muscles myotomes - on the vertebral column, a method by which a large muscle mass produces a relatively
small amount of action. This type of movement is efficient in a water medium, but it would be less
effective on land. The myotomes consist of blocks of longitudinal muscle fibers placed on each side of a
central axis, the vertebral column. Their contraction, therefore, bends the body, and the action passes
in waves down the body, alternating on each side.
After cutting off the sharp dorsal and ventral spines, scrape away the scales using your scalpel skin
one side of the body. Use forceps to lift the skin as you carefully separate it from the myotomes whit
the scalpel. Note the shape of the myotomes.
They resemble W's that are turned on their sides and stacked together. A horizontal septum of
connective tissue divides the muscles into dorsal epaxial muscles and ventral hypaxial muscles
(Fig. 10.4). Posteriorly both epaxial and hypaxial muscles are active in locomotion, but anteriorly the
hypaxial muscles serve more for support of body viscera than for locomotion. Try to separate some of
the myotomes. Observe the direction of the muscle fibers. Do they run zigzag as the myotomes seem
to? Or are they all directed horizontally - or vertically?
Dissection of individual muscles in the fish is difficult and will not be attempted here.
------------------------------------------------------Mouth cavity, pharynx and respiratory system. Before starting the dissection, it is well, if
you have not already done so, to cut off the sharp dorsal and ventral fins to protect your hands.
Cut away the operculum from the left side, exposing the gill-bearing bars or gill arches.
How many gill arches are there? Note the double row of soft gill filaments borne on the posterior
side of the arch. It is in these filaments, containing capillaries from the branchial arteries, that exchange
of gases takes place. Bony gill rakers on the oral surface of each gill bar, strain out food organisms
and offer some protection to the gill filaments from food passing through the pharynx.
Cut through the corner of the left side of the mouth and continue the cut through the middle of the
left gill arches to expose the mouth cavity and pharynx.
Open the mouth wide and note the gill slits in the pharynx. In the mouth, locate the fine teeth.
Would they be effective in chewing? In holding the prey? Just inside the teeth, across the front of both
the upper and lower jaw, are the oral valves. These are transverse membranes that prevent the outflow
of water during respiration. An inflexible tongue is attached to the floor of the mouth. Explore the
spacious pharynx, noting the size and arrangement of the gill bars and gill slits.
The mechanics of water movement involve a continuous pumping action in which, by muscular
action, the gill arches are pushed out laterally, and the opercula are pressed against the body; this
enlarges the branchial cavity, at the same time closing its exit, so that water flows into the mouth and
pharynx. Then, by closing the flap-like oral valves, opening the gill covers, and compressing of the gill
chamber, water is forced out over the gills, allowing for gas exchange at the gill filaments. These
breathing movements may be seen in the live fish on demonstration today.
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------------------------------------------------------DEMONSTRATION
Observe the living fish in the large aquarium. Watch a fish as it swims. How does the body move
to propel itself forward? Which fin(s) is used primarily for propulsion? What seems to be the functions
of the paired fins?
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Observe the breathing movements of the fish. Are the movements of the mouth and operculum
synchronized?
------------------------------------------------------Abdominal cavity. Starting near the anus and, being careful not to injure the internal organs,
use scissors to cut anteriorly on the midventral line to a region anterior to the pectoral fins. On the
animal's left body wall, make a transverse cut, extending dorsally from the anal region; curve this cut
anteriorly once the incision is halfway up the body wall. Continue to cut through the body wall and
into the gill chamber; then remove the left body wall by a gentle tug. Figure 10.5 shows a cutting
diagram. Wash the preserving fluid out of the body cavity by running tap water into it.
You have now exposed the abdominal or peritoneal cavity which contains the digestive tract
and glands, the reproductive organs and the swim bladder. This, together with the more anterior
pericardial cavity, which contains the heart, combine to make up the coelomic cavity, a term
which is derived from the embryology of the animal. Note the shiny lining of peritoneum which
covers the inner body wall and all the organs.
Probably the first organ you will see will be the intestine encased in yellow fat. Carefully remove
enough of the fat to trace the digestive tract anteriorly. Find the stomach, lying dorsal and somewhat
to the left of the intestine. Anterior to the stomach is the liver, dark red in life but bleached cream
coloured by preservative. The spleen is a slender dark-gray organ lying between the stomach and the
intestine. The gonads are in the dorsoposterior part of the cavity. Females often have a greatly
enlarged ovary, distended with eggs. Males have a pair of ivory coloured testes dorsal to the gut. The
air bladder lies dorsal to these organs and to the peritoneal cavity. It is a long and thin walled airfilled bag. Do not injure its walls or any of the blood vessels lying among the viscera. The kidneys are
located dorsal to the air bladder and will be seen later.
Digestive system. Run your probe through the mouth and into the opening of the esophagus
at the end of the pharynx. Now lift up the liver and trace the esophagus from the pharynx to the large
cardiac portion of the stomach. This ends posteriorly as a blind pouch. The short pyloric portion of
the stomach, opening off the side of the cardiac pouch, empties by way of a pyloric valve into the
duodenum, the S-shaped proximal part of the intestine. Three intestinal diverticula, the pyloric
caeca (singular: caecum), open off the proximal end of the duodenum near the pyloric valve. Follow
the intestine to the anus. Note the supply of blood vessels in the mesentery. The pancreas, a rather
indistinct organ, lies in the fold of the duodenum. The liver is large and lobed, with the gallbladder
located under the right lobe. The liver is drained by tubules into the gallbladder, which in turn opens
by several ducts into the duodenum posterior to the pyloric caeca.
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Air bladder (swim bladder). The air bladder is a long, shiny, thin-walled sac that fills most of
the body cavity dorsal to the visceral organs. In some fishes (not the perch) it connects with the
alimentary canal. In living specimens the swim bladder in gas or air-filled and serves as
a buoyancy organ. Alterations of the volume of gas within the swim bladder assist the fish in
compensating for the differences in the specific gravity between its body and that of the surrounding
water while it is swimming at different depths.
Reproductive system. The sexes are separate, but it is very difficult to distinguish them
externally. It is best to confirm the sex by looking internally at the gonad.
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Internally, in the female the ovary is single and lies back of the stomach, just below the air
bladder and dorsal to the intestine. The size of the ovary varies seasonally, being largest during the
winter months prior to spawning. A prolongation of the ovary posteriorly serves as a sort of oviduct
for carrying eggs to the urogenital pore just posterior to the anus.
In the male two elongated testes are attached to the air bladder by mesenteries. They become
greatly enlarged prior to spawning and are usually smallest during the summer months. A sperm duct
(vas deferens) runs along a longitudinal fold in each testis adjacent to the spermatic artery. The two
ducts join in the posterior midline and extend to the genital pore just posterior to the anus.
Excretory system. The kidneys are paired masses that lie against the dorsal body wall and
extend the whole length of the abdomen above the air bladder. They are often fused posteriorly, but
the anterior parts are usually separated by the dorsal aorta. The anterior ends consist largely of blood
sinuses and have lost their renal function. In the posterior end they follow the body wall ventrally.
Here the posterior ends ducts may be seen extending the short distance from the kidneys to the small
urinary bladder, which lies posteriorly between the gonad and the air bladder. In the female the
urinary bladder joins the oviduct to form a urogenital sinus, emptying through the urogenital pore.
In the male the bladder empties separately through a urinary pore, around which there may be a small
external projection of the bladder called the urinary papilla. The male urinary and genital pores lie
close together posterior to the anus.
Circulatory system. Extend the midventral incision to the jaw to expose the heart and
pericardial cavity. Enlarge the opening by removing a triangular piece of body wall on each side of
the cut.
The pericardial cavity is separated from the abdominal cavity by a transverse septum. The septum
is not homologous to the diaphragm of mammals.
The heart has two chambers - a thin-walled atrium and a muscular ventricle. Blood collected
from the venous system enters the sinus venosus, a thin-walled sac adjoining the atrium posteriorly.
Blood flows from this into the atrium and from there to the ventricle, which lies ventral to it and is the
most prominent part of the hart. The ventricle pumps into a short swollen bulbus arteriosus, which
is really the first part of the ventral aorta.
From the bulbus arteriosus, the blood flows into the short ventral aorta. Remove the operculum
and trace the aorta forward. It gives off four pairs of afferent branchial arteries to the branchial arches
which branch to send capillaries into each gill filament. From the capillaries in the gills the oxygenated
blood is collected by efferent branchial arteries and is emptied into the two roots of the dorsal aorta
above. These roots join immediately to form the dorsal aorta which carries oxygenated blood to the
body of the perch. The various branches of the dorsal aorta carry arterial blood to the capillary beds of
the body tissues where gas, waste and nutrients are exchanged. All blood returning to the heart via the
veins passes first through the sinus venosus before going to the atrium. Be sure you can trace the
pathway of circulation in the Perch.
------------------------------------------------------LABORATORY ASSIGNMENT 2
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Use your dissected specimen to assist you to completely label the diagram of the internal structure
of the perch. DO NOT discard your specimen until you have completed this assignment.
------------------------------------------------------LABORATORY ASSIGNMENT 3
Answer the following questions.
1. On which surface of the fish do you find the anal fin? __________________________
2. On which end of the fish do you find the nostrils? _____________________________
3. What plane would you cut through to divide the fish into identical left and right halves?
____________________________________________________________________
4. Circle the correct term:
The anal fin is anterior/posterior to the pectoral fin.
On your arm, your fingers are distally/proximally placed.
The pectoral fins are attached laterally/medially.
5. a. Which are the paired fins? ___________________________________________
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b. Name the three unpaired fins. _______________________________________
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c. What is the function of each of the fins? ________________________________
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6. Describe the movements of the mouth and the operculum as related to the water circulation over the
gills. __________________________________________________________
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____________________________________________________________________
Describe the circulatory pathway of blood traveling through the fish. __________________
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