Download Lab 10- FishDissection

10- 1
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.
10- 2
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.
10- 3
Distal - away from the center or point of attachment; the opposite of proximal.
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.
10- 4
----------------------------------------------------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.
-----------------------------------------------------
10- 5
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
10- 6
the next one. The scales grow continuously during the life of the fish and are not regenerated
if lost. Seasonal variations in the growth of the fish are reflected in the rings on the scales and
can be examined microscopically to determine the age of the fish.
________________________________________________________
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.
10- 7
10- 8
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.
-------------------------------------------------------
10- 9
10- 10
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.
10- 11
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.
-------------------------------------------------------
10- 12
------------------------------------------------------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?
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 air-filled 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
10- 13
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.
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.
10- 14
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.
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
10- 15
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
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? ___________________________________________
____________________________________________________________________
b. Name the three unpaired fins. _______________________________________
____________________________________________________________________
c. What is the function of each of the fins? ________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
10- 16
6. Describe the movements of the mouth and the operculum as related to the water circulation
over the gills. __________________________________________________________
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
Describe the circulatory pathway of blood traveling through the fish. __________________
____________________________________________________________________
____________________________________________________________________
10- 17
10- 18