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Zoology Exercise #18: Chordates: Fish Lab Guide Chordates show
remarkable diversity. Most are vertebrates. All animals that belong
to this phylum MUST, at some point in their life cycle, show the
following characteristics:
1. Notochord: slender rod of cartilage – like tissue lying near
the dorsal side and extending most of the length of the animal. (in
most vertebrates, it’s only found in embryos)
2. Pharyngeal pouches/slits: series of paired slits in the
pharynx, serving as passageways for water to the gills. (in some
vertebrates, it’s only found in embryos)
3. Dorsal nerve cord: with/modified portions that are identified
as a brain, form the central nervous system. Normally lies dorsal
to the digestive tract.
4. Endostyle or thyroid gland: found in all chordates, but not
other animals. Normally secretes mucous and traps small food
particles (for early vertebrates).
5. Postanal tail: usually projects past the anus at some stage
and serves as a means of propulsion in water. May or may not
persist in the adult form.
Exercise 18A: Subphylum Cephalochordata – Amphioxus The little
lancelet, also called Amphioxus, demonstrates the basic chordate
structure. This animal is common along the southern California and
Atlantic coasts of the United States. On sandy bottoms it will dive
head first, then twist upward so the tail remains buried in the
sand and the anterior end is thrust upward into the water. Anatomy
The animal has a dorsal fin which will broaden in the tail region
and become the ventral fin. At the anterior tip is the rostrum.
These animals have an oral hood which is fringed by tentacles (also
called buccal cirri). These will strain out large particles of sand
and are sensory in function. On the ventral surface is the
atriopore, which is anterior to the ventral fin. It is the opening
to the atrium, which is a large cavity surrounding the pharynx. The
anus will open slightly to the left of the posterior end of the
ventral fin. In mature specimens, block – like gonads will lie in
the atrium, anterior to the atriopore. They can be seen through the
body wall. Segmentally arranged muscles called myotomes can be seen
in cross sections. Beginning right above the oral hood is the
notochord, which may help spread the hood open. The mouth is
difficult to see, but is a very small opening right behind the
cavity created by the oral hood. It leads to the pharynx. Within
the cavity is several, finger like ciliated patches that compose
the wheel organ. This organ will rotate and help to maintain a
current of water flowing into the mouth. The large pharynx will
narrow into a straight intestine extending to the anus. The
sidewalls of the pharynx are composed of a series of gill bars,
between which are pharyngeal slits. Just posterior to the pharynx
is the hepatic cecum (liver) which extends forward along one side
of the pharynx. Surrounding the pharynx is the atrium which extends
to the atriopore. Water enters the mouth, filters through the gill
slits, into the atrium and then out the atriopore. These animals
are filter feeders with the ciliated tentacles, wheel organ, and
gills drawing in a steady current of water that is loaded with
food. This food is sorted of unwanted particles. On the floor of
the pharynx is an endostyle which has ciliated cells that secrete a
mucus. Food entangled in the stream of mucus is carried upward by
cilia on the inner surface of the gill bars, then backward toward
the intestine. Digestion occurs in the intestine. Oxygen – Carbon
Dioxide exchange occurs in the epithelium covering the gills bars.
The notochord helps to provide skeletal support and a point of
attachment for muscles. Above the notochord is the dorsal tubular
nerve cord. It contains pigmented photoreceptor cells.
Chemoreceptors are scattered over the body but are abundant on the
oral tentacles. Touch receptors are located over the entire body.
These animals do not have a heart but peristaltic contractions of
the ventral aorta keep the colorless blood in motion, sending it
forward and then upward through arteries, to capillaries, and then
to the gill bars for gas exchange. Blood is carried from the gills
to a pair of dorsal aortas. Then to segmented arteries to
capillaries of the myotomes and to capillaries in the wall of the
intestine. From the intestinal wall nutrients pass into the blood
where veins will carry it to various parts of the body, then back
to the ventral aorta. Nephridial tubules will be used to carry and
remove waste.
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Procedure
1. Now, examine with low power, a stained whole mount and cross
section of Amphioxius. Identify the following: dorsal fin, tail
region (caudal fin), ventral fin, rostrum, myotomes, oral hood,
pharynx, wheel organ, intestine, gill bars, pharyngeal slits,
intestine, anus, hepatic cecum, atriopore, notochord, dorsal nerve
cord, endostyle
SKETCH BEFORE YOU BEGIN EXERCISE 18B, you will need to define
the following terms and label the diagram below. From this point
forward, these terms might be used as you dissect. You must be
familiar with these terms to avoid making any mistakes which might
result in you now being able to determine various anatomical parts.
BRING IT UP TO BE CHECKED WHEN FINISHED! Define each of the
following: Lateral: Medial: Proximal: Distal: Dorsal: Ventral:
Anterior (Cranial): Posterior (caudal): Superficial: Deep: Sagittal
plane: Midsagittal plane: Transverse plane: Frontal plane:
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Next, label the diagram below: 1.
________________________________________
2. ________________________________________
3. ________________________________________
4. ________________________________________
5. ________________________________________
6. ________________________________________
7. ________________________________________ (plane)
8. ________________________________________ (plane)
9. ________________________________________ (plane)
Exercise 18B – Class Petromyzontida – Lamprey Hagfish and
Lampreys are grouped together into a superclass called Agnatha.
They share certain characteristics including; absence of jaws, no
internal ossification (bone), no scales, and no paired fins.
Lampreys are anadromous, meaning they ascend rivers and streams to
spawn. They have a habit of holding on to its position by grasping
with its mouth. It can grow to be 1m long and can live in both
freshwater and the sea. If a marine species, it will migrate up
freshwater streams to spawn. Young larvae are known as ammocoetes.
These live in the sand for 3-5 years as filter feeders. They
metamorphosize rapidly into adults and become parasites of fishes.
They attach by their sucker-like mouth and rasp away the fish’s
flesh with their horny teeth and suck out blood and body fluids.
Adults will grow rapidly for a year, spawn in winter or spring,
then die. Freshwater lampreys have habits that are similar to
marine species, but many are not parasites and many adults do not
eat and only live a month or so. Just long enough to spawn.
External Anatomy The adult lamprey has an eel like shape with a
tough, scaleless skin. There are numerous gland cells that produce
a protective slime. It has two dorsal fins and a caudal (tail) fin.
There are no paired appendages. It has a hood shaped buccal funnel
(cavity) supported by a cartilaginous ring that serves as a sucking
disc for attachment to its host. The opening is fringed by numerous
fingerlike sensory buccal papillae. The interior of the funnel
bears horny epidermal teeth. The mouth is at the back of the buccal
funnel and dorsal to the tongue. A single nostril is located
mid-dorsally on top of the head and opens into an olfactory sac.
Just behind the nostril is a small, oval area marking the position
of the third eye, the pineal organ. It is not a true eye, but does
contain photoreceptors that can detect light. Its eyes are lidless
and there are 7 external gill slits. The lateral line is located in
small patches on the head and trunk. They appear as pores and are
specialized receptors that are sensitive to currents and water
movement. The urogenital papilla is a projection at the juncture of
the trunk and tail. There is an anal opening in front of the
urogenital opening. Internal Anatomy The lamprey retains the
notochord and is a rodlike mass of cells enclosed by a tough,
fibrous sheath. It is firm yet flexible and prevents the body from
shortening when the muscles contract. Its skeleton also consists of
various elements of cartilage and fibrous connective tissue. The
digestive tract begins at the mouth (within the oral hood) and
continues into a pharynx. This pharynx leads into two tubes. The
esophagus which will continue into the intestine. Eventually ending
at the cloaca. There is no stomach. The second tube is the
(branchial) respiratory tube. It is perforated by 7 internal
gills.
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The brain and spinal cord are found above the notochord. The
nostril leads first into the olfactory sac. Water is drawn in and
squeezed out of the olfactory sac with each respiratory movement of
the pharynx. The heart lies within the pericardial cavity. The
sinus venosus receives blood from the body. This blood empties into
the atrium on the left side of the pericardial cavity. Blood passes
into the ventricle on the right side of the pericardial cavity and
is then pumped into the ventral aorta. The ventral aorta gives off
8 pairs of afferent branchial arteries that lead to gill
capillaries. The blood is then oxygenated and collected by the
dorsal aorta which lies just ventral to the notochord. The dorsal
aorta will then supply blood to the viscera and muscles.
Procedure
1. Examine a preserved specimen of an adult lamprey. Identify
the following: dorsal fin, caudal fin, buccal funnel, sensory
papillae, teeth, mouth, tongue, nostril, pineal organ, eyes, gills
slits, lateral line, urogenital papillae, anal opening.
SKETCH
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2. Now, create a sagittal section by using a scalpel to first
make a transverse cut through the first 1/3 of your lamprey nearest
the anterior end. Cut completely through. Now, take this 1/3 of
your lamprey and begin at the most anterior end where you find the
buccal funnel and cut the animal lengthwise. This will produce a
sagittal cut similar to what you see in the diagram above. Identify
the following parts: notochord, mouth, tongue, pharynx, esophagus,
intestine, respiratory tube, gill slits, brain, spinal cord,
nostril, olfactory sac, nasopharyngeal pouch, heart, ventral aorta,
dorsal aorta,
SKETCH
Exercise 18C – Class Chondrichthyes – Cartilaginous Fishes This
group contains about 970 species that are characterized by a
skeleton made of cartilage, powerful jaws, and well developed sense
organs. They include the sharks, skates, and rays. Most are
carnivores and many are top predators. The dogfish shark will be
our representative member. It is a small marine shark that grows to
about 1 meter in length. They are distinguished by a spine on the
anterior edge of both dorsal fins. These fish have the ability to
sense weak electrical fields in nature and use this to detect and
capture their food (small bottom dwelling fish and crabs). The
Ampullae of Lorenzini allows them to detect these electrical
fields. They are ovoviviparous, giving birth to live young without
the dependence on nourishment from the mother. Embryos develop in
an egg capsule in the oviduct until they hatch in the mother just
before birth.
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*** Please note that the procedure steps for this part of the
activity are embedded in the descriptions below. READ CAREFULLY!!!
YES, you will need to know the items in bold print for the
practical.
External structure
The body is divided into the head (anterior to the pectoral
fins), trunk (from pectoral fins to pelvic fins), and tail. The
fins include a pair of pectoral fins (anterior), which control
changes in directions during swimming; a pair of pelvic fins, which
serve as stabilizers and which in the male are modified to form
claspers used in copulation; two median dorsal fins, which also
serve as stabilizers; and an asymmetrical caudal (tail) fin. The
spiny dogfish is so named for a pair of spines immediately anterior
to each dorsal fin. These spines are often removed from dissection
specimens as they are mildly poisonous!
Identify the mouth with its rows of teeth (modified placoid
scales), which are adapted for cutting and shearing; two ventral
nostrils, which lead to olfactory sacs and which are equipped with
folds of skin that allow continual in-and-out movement of water;
and the lateral eyes, which lack movable eyelids but have folds of
skin that cover the outer margin of the eyeballs. The part of the
head anterior to the eyes is called the snout. A pair of dorsal
spiracles posterior to the eyes are modified gill slits that open
into the pharynx. They can be closed by folds of skin during part
of the respiratory cycle to prevent the escape of water. Five pairs
of external gill slits are the external openings of the gill
chambers. Insert a probe into one of the slits and notice the angle
of the gill chamber. The pharynx is the region in back of the mouth
into which the gill slits and spiracles open. A lateral line,
appearing as a white line on each side of the trunk, represents a
row of minute, mucus-filled sensory pores used to detect
differences in the velocity of surrounding water currents, and thus
to detect the presence of other animals, even in the dark. Note the
cloacal opening between the pelvic fins. This is a common exit for
digestive and urinary waste, sperm from the male reproductive
system and, in the female, the passageway through which the pups
are born.
The skin consists of an outer layer of epidermis covering a much
thicker layer of dermis densely packed with fibrous connective
tissue. The leathery skin is covered with placoid scales. Each
scale has a wide base embedded in the skin and a spine that
projects from the surface pointing posteriorly. Run your hand over
the skin, first from heat to tail, then back the other way to feel
the projecting spines of the scales. These are very different than
the scales of bony fishes. Placoid scales are actually similar in
structure to teeth. The dark dorsal and light ventral coloration of
the skin makes the shark less conspicuous
SKETCH
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Internal structure
Open the coelomic cavity by extending the mid-ventral incision
caudally to just in front of the cloacal opening and to just below
the mouth. You will need to cut through the cartilage of the
pectoral girdle between the pectoral fins. Now, make transverse
cuts caudal to the pectoral fins and cranial to the pelvic fins to
open the posterior part of the coelomic cavity. Rinse out the body
cavity with water.
The body cavity is lined with parietal peritoneum, a shiny
membrane tightly adhering to the inner surface of the cavity. Each
organ in the body cavity is also covered with a tightly adhering
membrane, called visceral peritoneum. These peritoneal membranes
come together to form the double-membraned dorsal mesentery that
supports the digestive tract.
Digestive system Identify the large liver which is very rich in
oil for energy storage. The liver has two large lobes and a small
median lobe. Note the elongated greenish gallbladder, embedded in
the median lobe. Move the liver aside to see the large esophagus,
which leads from the pharynx to the J-shaped stomach. Follow the
stomach around the curve of the ‘J’ and locate a narrowing point;
this is the pyloric valve, a muscular constriction between the
stomach and the duodenum (the first part of the intestine). The
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pyloric valve controls the passage of food out of the stomach.
Make a slit in the wall of the stomach and extend the cut upward
into the esophagus. Remove and examine the contents of the stomach
and then rinse it out to allow you to view the rugae (folds) within
the stomach and the papillae lining the inner wall of the
esophagus.
Next, find the 2 portions of the pancreas. A small portion sits
partially on the ventral surface of the duodenum, while a slender
dorsal portion extends posteriorly to the large, triangular spleen
(not a part of the digestive system) but does function to produce
red blood cells. Finally, identify the valvular intestine, a short,
wide tube which contains a spiral valve. Make a slit in the wall of
the valvular intestine and open the tube enough to view the
internal spiral valve. The spiral valve increases the surface area
for absorption of nutrients in this very short intestinal tube. The
valvular intestine narrows into the colon, which empties into the
cloaca. Locate the long, thin rectal gland, dorsal to the colon.
The rectal gland concentrates and excretes salt, important in
osmoregulation.
SKETCH (Digestive System) Urogenital system Although the
excretory and reproductive systems have very different functions,
they are closely associated structurally, and so are studied
together. The kidneys are long and narrow and lie behind the
parietal peritoneum ,one on each side of the midline of the dorsal
body wall. These long, narrow kidneys extend from the pectoral
girdle to the cloaca. Running along the surface of each kidney is a
convoluted wolffian duct which (in females) carries the urine
formed in the kidney to the renal papilla inside the cloaca for
excretion. Open the cloaca to see the renal papilla.
Male – Locate the testes along the dorsal body wall, one on each
side of the esophagus. A number of very fine tubules (too small to
see with the naked eye) connect each testis to the wolffian duct
(also called sperm duct in males). Sperm is formed in the testes
and then travels through the wolffian ducts to sperm sacs which
empty via the renal papilla into the cloaca. Thus the wolffian duct
in males carries sperm, not urine. Accessory urinary ducts receive
the urine formed in the kidneys and transports it to the renal
papilla and into the cloaca. The male pelvic fins include modified
structures called claspers. The claspers direct the sperm and
seminal fluid from the cloaca of the male to the cloaca of the
female during copulation.
Female - A pair of ovaries lies against the dorsal body wall,
one on each side of the esophagus. In mature specimens, enlarged
ova may form several rounded projections on the surface of the
ovaries. A pair of oviducts travel next to each kidney along the
dorsal length of the body cavity and enlarge at the caudal end to
form the uterus. At the cranial end, the oviducts join and have a
common opening called the ostium (the uterus and ostium are
difficult to see in immature specimens). When an egg ruptures
through the surface of the ovary into the abdominal cavity, it is
swept into the ostium and then into one of the oviducts.
Fertilization occurs inside the oviducts and the fertilized eggs
develop into embryos in the uterus. Amazingly, dogfish shark
embryos take almost 2 years to develop within the uterus and are
born live, exiting the uterus through the cloaca. This type of
development is termed ‘ovoviviparous’, meaning the young are born
live, but during gestation receive
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nutrients mainly from the egg, not directly from the mother’s
uterus. Human development is ‘viviparous’ – young are born alive
and receive nutrients via the mother’s uterus. SKETCH (Urogenital
System) Circulatory system - Heart The heart lies in the
pericardial cavity, cranial to the pectoral fins and the
cartilagenous pectoral girdle. The human circulatory system
consists of 2 separate circulation: the pulmonary circulation,
which pumps deoxygenated blood to the lungs and then receives the
oxygenated blood back from the lungs and the systemic circulation,
which pumps oxygenated blood to the entire body and receives the
deoxygenated blood back from the body. The shark has only a single
circulation and the heart pumps only deoxygenated blood through as
follows:
1. Deoxygenated blood returns to the heart via veins and enters
the thin-walled, flat sinus venosus (you will need to lift the main
portion of the heart to view this structure)
2. Blood flows from the sinus venosus into the atrium, which is
a thin-walled chamber with 2 lobes bulging out to the sides. The
atrium also is best seen by lifting the main portion of the
heart.
3. Blood flows next into the most obvious and muscular chamber,
the ventricle. The atrium and ventricle constitute the classic ‘2
chambered fish heart’.
4. The ventricle contracts to push the blood into the conus
arteriosus, a muscular tube which exits the ventricle cranially and
narrows into the ventral aorta. The ventral aorta is the main
ventral blood vessel in the head. Branches from the ventral aorta,
the afferent branchial arteries, carry the deoxygenated blood to
the gills, where oxygenation of the blood occurs.
Circulatory system - Arteries As mentioned above, the ventral
aorta and the afferent branchial arteries transport the
deoxygenated blood from the heart to the gills, for oxygenation. To
view these vessels you must remove a large amount of muscle tissue
from the ventral portion of the head up to the lower jaw. It is
best to do this dissection by carefully following the ventral aorta
forward as you remove the muscle tissue. Do this carefully so as
not to damage the underlying blood vessels. As you follow the
ventral aorta forward, look for vessels branching off to the sides
– these are the afferent branchial arteries, which deliver the
deoxygenated blood to the gills. Efferent branchial arteries
(difficult to dissect, so we will not see these) return the newly
oxygenated blood to other blood vessels which deliver the
oxygenated blood to all parts of the body.
You can easily locate two of the vessels which deliver blood to
the lower part of the body – the dorsal aorta and the celiac
artery. Both these vessels may have been injected with red plastic
to make them easier to observe. The dorsal aorta travels
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the length of the body and can be located between the kidneys.
Once you have found the dorsal aorta, look for the celiac artery -
a prominent branch from the aorta which travels via the mesentery
toward the organs in the abdominal cavity.
Circulatory system - Veins Look for a prominent vessel running
in the mesenteries from the intestines to the liver – this is the
hepatic portal vein. It may have been injected yellow. This vein
gathers blood chiefly from the digestive system and delivers this
nutrient rich blood to the liver, where the carbohydrates are
converted and stored in liver cells for future energy needs. We
will not locate any other veins, however be aware that the entire
body is served by a system of veins which return the deoxygenated
blood to the heart. SKETCH (Circulatory System)
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Respiratory system In the sharks, water enters through both the
mouth and the spiracles and is forced laterally through the five
pairs of gills and exits through the five pairs of external gill
slits. On one side, separate the gill units by cutting dorsally and
ventrally from the corners of each gill slit. Visualize the gill
chamber within which the gill is bathed by water rich in oxygen.
Continue to cut between adjacent gills and extract a portion of a
gill to examine. The incomplete rings of heavy cartilage supporting
the gills and protecting the afferent and efferent branchial
arteries are called gill arches. Short spikelike projections
extending medially from the gill arches are the gill rakers, which
filter the respiratory water and direct food toward the esophagus.
Examine the soft, brown tissue comprising the gill filaments – the
site of actual gas exchange. The pink color you see within this
tissue is due to the large number of capillaries. Oxygen absorbed
from the water diffuses into blood within these capillaries, just
as oxygen diffuses into capillaries in the alveoli of human lungs.
In addition, carbon dioxide diffuses out of the blood and into the
water within the gill chambers, where it exits through the external
gill slits.
SKETCH (Respiratory System)
Nervous System
Remove the skin from the dorsal surface of the head and shaving
off thin horizontal chips of cartilagenous cranium until the brain
and cranial nerves are exposed. Use your scalpel to shave off one
millimeter thick sections so that you don’t cut into the brain or
nerves. The delicate vascular protective membrane called the
primitive meninx needs to be removed. The nervous system functions
in communication between the various parts of an organism and
between the organism and its external environment. It consists of
the central nervous system; the brain and spinal cord, and the
peripheral nervous system; the sense organs, cranial and spinal
nerves, and their branches.
A. Examine the dorsal view of the shark's brain. You should be
able to identify the following organs.
Olfactory Sacs – Two large bulbous nerve sensors that detect
chemicals in the surrounding water.
Olfactory Lobes – Area of the brain that receives nerve signals
from the olfactory sacs and processes them.
Cerebrum – The two hemispheres between the olfactory lobes and
are associated with sight and smell.
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Diencephalon – The region just caudal from the cerebrum and
separates the fore and mid- brain. Includes the thalamus and the
hypothalamus.
Optic Lobe – Large prominent lobes of the mid-brain that receive
nerves from the eyes.
Cerebellum – Just caudal from the optic lobes it controls
muscular coordination and position.
Medulla Oblongata – The base of the brain, a widening of the
spinal cord. Controls many of the spinal reflexes. SKETCH (Nervous
System) Clean-up Procedure
Put your specimen in the plastic bags provided and close with a
rubber band. Put your bagged specimen in your drawer.
Clean your dissection tray thoroughly with soap and water.
Use the pray cleanser to and paper towels to clean the tabletop
in your work area.
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Exercise 18D– Class Osteichthyes – Bony Fishes *** Please note
that the procedure steps for this part of the activity are embedded
in the descriptions below. READ
CAREFULLY!!! YES, you will need to know the items in bold print
for the practical. Fishes are the oldest VERTEBRATE group and the
most numerous and widespread of all living vertebrates today. 95%
of all fish are in the class OSTEICHTHYES meaning “bony fish”. All
bony fish have three characteristics: 1). an ENDOSKELETON made of
BONE 2.) LUNGS or a SWIM BLADDER 3.) a body surface covered with
SCALES
INTEGUMENTARY: The skin of the perch is covered with SCALES
(thin round discs of bonelike material that grow from pockets in
the skin). The scales overlap like roof shingles and point toward
the tail in order to reduce friction as the fish swims. Scales grow
throughout the fish’s life and the resulting growth rings give a
good approximation of the fish’s age. Scales also provide
protection. The fins on a fish are adaptations for swimming and
navigation and are supported by RAYS or SPINES which also provide
protection from predators.
The two DORSAL FINS (one anterior and one posterior) and a
ventral ANAL FIN help keep the fish upright and moving in a
straight line. The paired PELVIC FINS and PECTORAL FINS are used to
stop, move up and down, and even back up. The CAUDAL FIN extends
from the tail for propulsion. The ANUS and UROGENITAL OPENING are
located near the anal fin. NERVOUS (Sense organs) The LATERAL LINE
system, which runs along each side of the fish, is a sensory
structure which detects water pressure and vibrations in the
water. Find the NOSTRILS (dead end pockets) and EYES (with no
eyelids). Fish have a highly developed sense of smell and sight and
the parts of the fish’s brain that process info from these two
areas (OPTIC TECTUM and OLFACTORY LOBES) are the largest parts of a
fish’s brain. COLORATION: Pigment cells (CHROMATOPHORES) in the
skin give the fish its color and allow it to blend in with its
surroundings. Notice the fish has lighter coloration on its ventral
surface and is darker on the top so it is less easily seen from
above or below. RESPIRATORY/EXCRETORY: On each side of the head is
the OPERCULUM, a hard plate that covers and protects the GILLS.
Water enters through the fish’s mouth, passes over the gills, and
out through the slits behind the OPERCULUM. Water moving over the
gills flows away from the head, while the blood inside the gills
flows toward the head. This arrangement, known as COUNTERCURRENT
FLOW (a.k.a RAM VENTILATION), allows more oxygen to diffuse into
the gills than would be possible if blood and water both flowed in
the same direction.
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The gills in a fish serve three functions: 1. EXCHANGE OF GASES
(oxygen is taken in and carbon dioxide is released), 2. REMOVAL OF
NITROGEN WASTE (AMMONIA is removed from blood and released into the
water 3. OSMOREGULATION OF WATER/ION CONCENTRATION IN BLOOD (IONS
are actively transported IN or OUT depending on environment) In
order to stay alive an organism must keep the balance of ions and
water in a constant range. This is done through a process called
OSMOREGULATION, which means maintaining the proper balance of water
and ions in the blood and body tissues. FRESHWATER FISH: Freshwater
fish tend to GAIN WATER and LOSE IONS in their HYPOTONIC
environment. The gills in a perch (freshwater dweller) have special
cells that ACTIVELY TRANSPORT sodium and chloride ions in through
the gills to maintain the correct ion balance. The KIDNEYS also
remove excess water by making urine. Freshwater fish urinate
constantly to remove the excess water that is always entering their
bodies from their hypotonic environment. SALTWATER (MARINE) FISH:
The reverse happens in SALT-WATER fish. Since sea water is
HYPERTONIC, water is constantly leaving the fish’s body via osmosis
and ions are entering through diffusion. To maintain the water/ion
balance, salt water fish urinate less and drink sea water to
replace lost water. They excrete the extra ions taken in through
special cells in their gills that maintain the proper osmotic
concentration in their blood and tissues. Extra ions are also
excreted in urine. SKETCH (External Anatomy)
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INTERNAL ANATOMY: Use your scissors to slice along the ventral
surface and peek inside to see the SWIM BLADDER (also called
AIR/GAS BLADDER). This organ is thought to have evolved from the
lungs of early bony fish. Gases (oxygen, carbon dioxide, and
nitrogen) from the blood can be added to or removed from the swim
bladder to control the fish’s buoyancy. By adjusting the volume of
gas in the swim bladder, a fish can remain suspended at any depth
with no muscular effort. MUSCULAR/SKELETAL Fish are “top heavy”
with muscle because the body muscles are concentrated along the
dorsal surface and in the tail of your fish. (One of the reasons
fish float “belly up” when they are dead). An ENDOSKELETON of bone
provides support and helps in movement. Having an endoskeleton
allows a vertebrate to grow without molting. Bones (called
vertebrae) surround their SPINAL CORD, as well. **** Place your
fish on its RIGHT SIDE and remove the body wall on the left side of
your fish so you can see the internal organs. The space you see
surrounding the organs is true COELOM. Notice the location of the
liver, gills, and heart. It is no accident these vital organs are
so close together. REPRODUCTIVE Fish have SEPARATE SEXES. The male
reproductive system consists of paired TESTES that produce sperm
which are carried by the VAS DEFERENS to the shared UROGENITAL
OPENING that releases both urine and eggs or sperm. In females eggs
are produced in paired OVARIES and carried via OVIDUCTS to the
UROGENITAL OPENING. Eggs and sperm are released through this
urogenital opening behind the ANUS. Most fish have EXTERNAL
FERTILIZATION. The female lays eggs and the male passes over them,
depositing the sperm to fertilize them. Mortality among eggs and
young is high and fish lay large numbers of eggs to ensure at least
some will survive. Immature fish that hatch are called FRY. Many
fish display complex reproductive behaviors (SPAWNING) for
courtship, nest building, migrating, and caring for young.
DIGESTIVE Examine the MOUTH and PHARYNX (opening to the digestive
system in the back of the throat). The ESOPHAGUS is a short
muscular tube that connects the pharynx and the STOMACH which
produces acid and some digestive enzymes to begin the breakdown of
food. The CARDIAC STOMACH is closest to the mouth. The PYLORIC
STOMACH connects to the INTESTINE. The PYLORIC CAECA pouches
located near the junction of the pyloric stomach and the DUODENUM
(lst part of INTESTINE). Villi (fingerlike extensions along the
inside surface of the intestine) help to increase surface area for
better nutrient absorption by the intestine. The pyloric caeca are
believed to be involved in digestion of plants and absorption of
nutrients.
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Digestive waste moves through the intestine and exits the body
through the ANUS. The reproductive organ and KIDNEYS also exit in
this area through the shared UROGENITAL OPENING. The LIVER lies on
top of the STOMACH. It secretes bile (to help digest fats) which is
stored in the GALL BLADDER (darker tissue on the liver) until it is
used in the intestine. In addition to secreting bile, the liver
also functions in glycogen storage, vitamin storage, and processes
toxins (including nitrogen waste from the body cells) which are
then removed from the blood by the kidneys and gills (as AMMONIA).
The PANCREAS makes a digestive enzyme called TRYPSIN (that breaks
down proteins) which is released into the intestine. ENDOCRINE The
endocrine system controls sexual development, heart rate, and
metabolism. In addition to digestive enzymes (trypsin), the
PANCREAS makes two endocrine hormones that regulate blood sugar
levels. INSULIN causes cells to take up glucose from the blood
stream and store it as glycogen. GLUCAGON causes cells to release
their stored glycogen as glucose into the bloodstream. These to
hormones work together to control blood sugar levels. CIRCULATORY
The circulatory system in a fish delivers oxygen and nutrients to
the cells of the body. It also transports carbon dioxide and
nitrogen waste to the gills and kidneys for elimination. The
circulatory system consists of a heart, blood vessels, and blood.
Fish have a CLOSED circulatory system with blood contained in blood
vessels. The heart pumps blood in a single closed loop through
ARTERIES (vessels that carry blood away from the heart) to small
thin walled vessels in the gills called CAPILLARIES
where oxygen is picked up and carbon dioxide is released. From
the gills, blood travels to the tissues where nutrients and wastes
are exchanged via capillary walls. Blood returns to the heart in
vessels called VEINS. The HEART in a fish has 2 MAIN CHAMBERS: an
ATRIUM and a VENTRICLE. Deoxygenated (low oxygen) blood returning
to the heart empties into a collecting space called the SINUS
VENOSUS before moving into the atrium. Contraction of the atrium
speeds up the blood and drives it into the ventricle (main pumping
chamber). Contraction of the ventricle forces the blood through the
circulatory system. An exit space called the CONUS ARTERIOSUS
smoothes the flow of blood as it leaves the heart.
From BODY To GILLS The SPLEEN is a dark thin structure that lies
in the loops of the intestine near the cardiac stomach and
functions in red blood cell formation, destruction, and storage.
During times of low oxygen the spleen can release extra red blood
cells to carry more oxygen.
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EXCRETORY The KIDNEYS are dark colored tissue located on the
dorsal body wall inside the coelom. The function of the kidneys is
to remove nitrogen waste (ammonia and urea) from the blood that has
been produced and processed by the liver. ammonia, the major
nitrogen waste product, is highly toxic (poisonous) and must be
diluted with large amounts of water. The kidneys do this by making
URINE, which contains AMMONIA, IONS (like sodium and chloride) and
WATER. Urine is produced by kidneys and stored in the URINARY
BLADDER. Urine passes out through the UROGENITAL PORE behind the
anus. Remember sperm and eggs also use this shared opening! The
kidneys also function along with the gills in osmoregulation to
remove excess water that enters the body via osmosis and keep the
correct balance of ions in the blood and tissues. Freshwater fish
urinate constantly (up to 30% of their body weight daily) to remove
the excess water that is always entering their bodies due to the
HYPOTONIC environment in which they live. Marine (salt water) fish
have the opposite problem. Because they live in a HYPERTONIC
environment, water is always leaving a marine fish’s body. They
urinate very little and must drink sea water and actively excrete
the ions out through their gills in order to maintain their osmotic
balance. SKETCH (Complete Internal Anatomy)
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NERVOUS The nervous system in a fish includes the brain, spinal
cord, nerves that lead to and from all the parts of the body, and
various sensory organs. Fish are vertebrates with a dorsal nerve
cord running along the dorsal body wall. A nerve cord covered
with
bone is called a SPINAL CORD. The brain in a fish is more
complex than you have seen in invertebrates. The BRAIN consists of
several areas with different functions. Fish have a highly
developed sense of smell and sight and the parts of the fish’s
brain that process info from these two areas (optic tectum and
olfactory lobes) are the largest parts of a fish’s brain. The most
anterior part are the OLFACTORY LOBES (BULBS) which process info
for smell. The CEREBRUM is for higher thinking (learning, memory,
and problem solving) and integrates information from all the other
areas of the brain.
The largest part is the OPTIC TECTUM, which receives and
processes information from the fish’s visual, auditory {hearing},
and lateral line systems. The most posterior portions are the
CEREBELLUM (controls motor coordination & balance), and the
MEDULLA OBLONGATA (controls autonomic body organs and acts as a
relay station for information from sensory receptors throughout the
body). The SPINAL CORD is surrounded by vertebrae, extends along
the body, and carries nerve impulses to and from the brain. SKETCH
(Brain) Analysis
1. The Amphioxus you observed in 18A and the ammocoetes larvae
of the Lamprey look very similar. Below, discuss these similarities
and also list some differences. (Since you did not make
observations of ammocoetes in lab, you will need to do additional
research to answer this question)
2. What does the lack of paired fins in Lampreys suggest about
how these animals swim through their environment?
3. The pineal gland seen in many vertebrates, especially the
Lamprey, is often called the “third eye”. But, it’s not really a
true eye. What is meant by this statement?
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4. The lateral line system in fish is characteristic for nearly
all members of this group. Explain its use and why it might be so
useful to the lamprey.
5. Bony fish will pass water through the mouth and then over the
gills, eventually exiting behind the gills. How does this benefit
bony fish? This does not happen with Lampreys. Explain how this
fish might solve this problem.
6. What type of movement do the following fins provide to most
fish? a. Caudal fin:
b. Dorsal fin:
c. Pelvic fin:
d. Pectoral fin:
7. Sharks have placoid scales. What benefit does this type of
scale provide to the shark? Bony fish can have ctenoid scales (like
the perch), cycloid, or ganoid scales. Describe the differences
between each and give an example of a fish for each.
8. Sharks have an extremely large amount of oil in their
tissues. Why?
9. What does this spiral valve do for the shark? Human do not
have this spiral valve. How have we compensated? In other words,
how is the function of the spiral valve in sharks performed in
humans?
10. What are the advantages of a cartilaginous skeleton over
bone?
11. Sharks have “gill rakers”. Why?
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12. Explain how blood is oxygenated in the shark and create a
flow chart to show the complete path blood will take as it travels
through the animal. Use the circulatory system terminology
described in this lab as you answer.
13. Bony fish are often seen with 3 different types of mouth
orientations: Explain each and describe how a fish with that mouth
would feed.
a. Terminal:
b. Superior:
c. Inferior:
14. Sharks had a heterocercal tail, while bony fish tend to have
a homocercal tail. Sketch each below.
15. A large part of the mass in bony fish is the myomeres.
Describe how these myomeres appear and then explain how they assist
in movement
16. The swim bladder is a very effective buoyancy system or
hydrostatic organ. What does this mean? How does it compare to the
way a shark will maintain buoyancy? What might be a disadvantage to
having a swim bladder?