1. health and fitness
2. disease

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.
2.
3.
4.
5.
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)
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)
Dorsal nerve cord: with/modified portions that are identified as a brain, form the central nervous system.
Normally lies dorsal to the digestive tract.
Endostyle or thyroid gland: found in all chordates, but not other animals. Normally secretes mucous and
traps small food particles (for early vertebrates).
Postnatal 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 oral 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. On the roof of the pharynx is a ciliated hyperbranchial groove. 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.
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, atrium, atriopore, notochord, dorsal nerve cord, endostyle,
dorsal arota, ventral aorta,
BEFORE YOU BEGIN, 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:
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
supported by a cartilaginous ring that serves as a sucking disc for attachment to its host. The opening is fringed by numerous
fingerlike sensory papillae. The interior of the funnel bears horny 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 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 respiratory tube. It is perforated by 7 internal gills slits. Each gill slits leads into an enlarged branchial
pouch that is lined with gill lamellae.
The brain and spinal cord are found above the notochord. The nostril leads first into the olfactory sac with a much folded
inner surface and then into the elongate nasopharyngeal pouch. 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 vensosus 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.
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,
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.
*** 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
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
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.
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
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.
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.
2.
3.
4.
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)
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.
Blood flows next into the most obvious and muscular chamber, the ventricle. The atrium and ventricle constitute the
classic ‘2 chambered fish heart’.
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
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.
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.
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.
• 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.
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 spray cleanser to and paper towels to clean the tabletop in your work area.
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.
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.
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.
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.
To GILLS
From BODY
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.
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.
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 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.
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?
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?
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?