Download 2006S Bio153 Lab 8: Comparative Vertebrate Morphology Aug 8th

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2006S Bio153 Lab 8: Comparative Vertebrate Morphology
Aug 8th / Aug 10th
Comparative vertebrate morphology is the study of the evolution of form in vertebrates.
Because form and function are closely related, it overlaps with comparative physiology
(the study of the evolution of function). As usual, the guiding theme in the discipline of
biology is evolution. Our limbs are modified fins; our brains are convoluted swellings at
the end of the neural tube; our lungs arose from simple pouches of the pharynx. Thus,
by comparing structures and functions among groups of organisms, we can understand
the evolutionary history that creates patterns of diversity, and the adaptations that the
array of diversity represents. Furthermore, because we can see so many features of
ourselves in other groups of organisms, we can use them as model animals for
understanding our own anatomy and physiology.
The phylum Chordata has three subphyla: Urochordata (tunicates),
Cephalochordata (amphioxus), and Vertebrata (vertebrates). Superficially, these
groups show little resemblance to each other. However, they all share a common body
plan with four basic features. These features are:
1. Notochord. A notochord is a long rod that runs the length of the body. The
notochord is made up of cells and fluid
surrounded by a sheath of fibrous
tissue. Recall that a fluid-filled cavity can
act as a skeleton due to the
incompressibility of fluids. The fluid core
makes the notochord flexible, but
incompressible. Thus, it acts as a brace
against which body wall muscles can act
to move the organism. In vertebrates,
the notochord is replaced by the
vertebral column during embryonic
development. (In mammals, it is retained
only as a small gel-like core in each intervertebral disc). Early vertebrates
usually had a large notochord as well as a vertebral column.
2. Dorsal, hollow nerve cord. The nerve cord is formed through the folding of a
special region of ectoderm called the neural plate, and is located dorsal to the
notochord.
3. Pharyngeal slits. During the embryonic
development of chordates, the walls of the pharynx
(the part of the alimentary canal just behind the
mouth) form arches, which are pierced (or nearly
pierced) by slits. These slits are sometimes called "gill
slits", but this term is incorrect. While it is true that in
some chordates, such as fish, these pharyngeal slits
persist, the actual gills are the highly vascularized
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structures found along the edges of these slits. Respiration occurs along the gills,
not the slits. In urochordates and cephalochordates, the slits filter food from water.
The photograph above shows pharyngeal slits in the early human embryo.
4. Post-anal tail. Chordates share the presence of an extension of the body
(notochord and nerve cord and, in vertebrates, the vertebral column) past the anus.
A) Cephalochordates: Cephalochordates are a group of marine organisms that live in
warm temperate and tropical waters. They are typically small (< 5 centimeters in
length) and have a fish-like body shape. Branchiostoma lanceolatum (a.k.a.
amphioxus) is a sedentary animal that burrows into the sandy bottom with only the
anterior end protruding. It is a
filter-feeder, straining microscopic
dorsal hollow nerve cord
food particles from the sea water as
notochord
it passes through ciliated gill slits in
the pharynx. Its notochord is
intestine
myomeres
unusual compared to other
chordates in that it contains muscle
cells that connect directly to the
dorsal nerve cord. On demonstration
are slides of a whole mount and a
pharyngeal
gill arches
cross-section view of Branchiostoma
post-anal tail
lanceolatum. Observe the dorsal
hollow nerve cord, the
notochord, the pharyngeal gill arches and the post-anal tail.
B) Vertebrates are the largest and most diverse group of chordates (the majority of
vertebrates are bony fish). Because vertebrates’ internal skeleton of cartilage or bone
fossilizes well, their evolutionary history (which spans about 544 million years!) is well
described. They are characterized by the presence of a vertebral column, a series of
elements made of bone or cartilage called vertebrae (singular vertebra) that form
around and typically replace the embryonic notochord. The spinal cord is housed within
the vertebral column. The other major innovation that evolved in vertebrates is the
cranium – a structure of bone and cartilage that supports the sense organs and encloses
(or partially encloses) the brain.
Early embryonic development in a vertebrate: In the gastrula, surface ectoderm
thickens into a strip called the neural plate. As the
neural plate thickens, it is rapidly elongating, which
helps create the neural folds and eventually the
neural tube. To see how this works, simulate this
neural fold
elongation process on the sheet of plastic on
neural plate
demonstration. Grasp both ends and pull gently; a
somites
fold should form at the midline. In the vertebrate
embryo, these folds meet and fuse, forming the
neural tube. The neural tube eventually becomes the
brain and spinal cord. Somites are clumps of
mesoderm that give rise to the notochord, and
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eventually to muscles, the dermis, and the axial skeleton. The notochord provides
support early in development, but is replaced by the vertebral column. On
demonstration are slides of a 33h chick embryo. Identify the neural fold, neural
plate and somites.
The vertebrate skeleton: The skeleton is divided into the axial and the
appendicular skeleton. The axial skeleton includes the skull, or cranium, which
houses the brain and sense organs; the vertebral column, which provides rigidity and
support; and the ribs, which provide sites for muscle attachment, strengthen the body
wall and protect many vital organs. The appendicular skeleton is formed by the
girdles (pectoral & pelvic) and the limbs. The pectoral girdle attaches the forelimbs to
the axial skeleton; the pelvic girdle attaches the hind limbs to the axial skeleton.
1. The Cranium: The anterior portion of the skull houses olfactory and taste receptors
and often teeth; the posterior portion houses the brain and auditory apparatus and
attaches to the vertebral column, and the middle portion contains the visual receptors.
The skull is formed from many bones joined at sutures; in birds the sutures are rarely
visible. The foramen magnum is the large hole at the bottom of the skull leading into
the braincase. On the edges are projections called occipital condyles, which articulate
with the first vertebra in the neck. (A condyle articulates with an adjoining bone, while
a process is the site of muscle attachment.) The size of the braincase varies a great
deal among vertebrates. In general, carnivores have a larger braincase for their body
size than do herbivores. Birds have a thinner and lighter cranium than mammals.
Hearing: The middle ear consists of bones and an eardrum which transmit vibration.
These bones are derived from bones that were present in the lower jaw in ancestral
vertebrates. The bones of the middle ear (ossicles) will not be present in the lab
specimens as they fall out during preparation. Mammals have three ossicles, while birds
have just one: hearing is less developed in birds than in mammals.
Vision: Forward-facing orbits (eye sockets) suggest that the animal has binocular
vision, which enable it to judge distances well. Lateral-facing orbits provide good widefield vision (allow a wide area to be scanned). Birds have very large orbits; vision is the
main sensory modality in birds.
Jaws: The separation of food and air passages in the snout has been a major
evolutionary development in the vertebrates. The roof of the mouth is (the secondary
palate) helps in this function. In the nasal passages are the turbinate bones, which
look honeycombed. These are covered with the nasal mucosa in the living specimen.
In mammals, the articulation of the lower jaw with the skull indicates the animal’s
feeding ecology. In
herbivores (the skull on
the right), the
mandibular condyle
joins the lower jaw to
the skull at a point
above the level of the
= coronoid process
= temporals
teeth. In carnivores
= mandibular condyle
= masseters
= angular process
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(the skull on the left in the above figure), the lower jaw is less curved and the jaw
articulates in the same plane as the teeth. The difference is related to the attachment
of the two main muscles that control jaw closing. The masseters run from the
zygomatic arch (the bottom of the orbit) to the angular process (at the bottom of
the lower jaw). These are important in grinding food. The temporals run from the
coronoid process (which extends above the mandibular joint) to the back of the skull.
These provide crushing strength. In birds, the lower jaw contains four bones.
Teeth: Mammals are heterodonts, meaning they have different types of teeth
specialized for different functions. They are the only group of animals that chew their
food. In some mammals, however, the teeth have reverted to the primitive homodont
form (e.g. in dolphins and armadillos), or are absent (e.g. anteater). Incisors are the
front teeth; they are chisel-shaped and used for nipping. In herbivores, canines are
reduced or absent, but are prominent in carnivores, and are used for piercing and
tearing flesh. In herbivores there is a prominent gap called a diastema between the
incisors and the premolars. This gap accommodates the large tongue, which is
important in mastication. Premolars and molars are called cheek teeth; in herbivores
they are large and flat for grinding. In carnivores, specialized premolars and molars are
called carnassial teeth. They function like scissors, sliding past each other to slice
flesh. Omnivores, not surprisingly, have elements of both types of dentition. Mostly,
their teeth resemble carnivores, but the back molars of an omnivore a flat for grinding
plant matter, while the back molars of a carnivore are pointed and sharp. Modern birds
do not have teeth; grinding of the food is done in a specialized structure in the digestive
tract called a gizzard.
Examine the skulls on demonstration and identify the following features: foramen
magnum, occipital condyles, orbits, zygomatic arch, turbinate bones,
mandibular condyle, angular process, coronoid process, incisors, canines,
diastema, molars, carnassial teeth.
2. The appendicular skeleton: The appendicular skeleton of the bird shows several
modifications for flight. The bones are thin and hollow, and many bones are fused to
form a sturdy frame to anchor muscles during flight. Air spaces in the bones can from
part of the respiratory system. The sternum is greatly enlarged and “keeled” for the
attachment of the enormous pectoral muscles (which may make up 30% of a bird’s
body mass). The collarbones are fused (the “wishbone” or furcula) to act as a spring,
bending and recoiling during flight. Energy stored in the bent bone can be recovered
and used for thrust. Examine the appendicular skeleton of the cat, rat and pigeon.
Note the modifications for flight in the pigeon: hollow bones, fused collarbones,
keeled sternum.
You are not responsible for memorizing the names of all the bones on the
following diagrams, but you may be interested to see the homologous
structures in these two vertebrates.
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Dissection of the rat (Rattus norvegicus): Work in pairs to do these dissections.
Terms in bold are fair game on the final exam.
Glossary of Terms
Dermal: relating to the skin
Longitudinal: lengthwise
Dorsal: toward the back
Ventral: toward the belly
Lateral: toward the sides
Median: near the middle
Anterior: toward the head
Posterior: toward the hind end (tail)
Superficial: on or near the surface
Deep: some distance below the surface
Sagittal: relating to the midplane which bisects the left and right sides
Transverse: relating to the plane separating anterior and posterior
Horizontal: relating to the plane separating dorsal and ventral
Proximal: near to the point of reference
Distal: far from the point of reference
Caudal: toward the tail end
Pectoral: relating to the chest and shoulder region
Pelvic: relating to the hip region
Right & Left: refers to the specimen's right and left, not yours
Abdominal Cavity: related to the area below (posterior) the ribcage
Thoracic Cavity: related to the area above (anterior) the ribcage
External Anatomy
The trunk is divided into an
anterior thorax and a
posterior abdomen. Teats or
nipples, the external
openings of the mammary
glands, are located on the
ventral surface of the trunk.
There are usually 12 in the
rat. Most mammals have
separate urogenital and anal
openings. In female rats, the
urinary (urethral) and genital
(vaginal) openings are also
separate, with the anal
opening dorsal to them. In
males, the urogenital
structures consist of the
penis, and a double pouch,
the scrotum containing the
testes.
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Internal Anatomy
1. Pin the rat by its paws to the wax in the tray so that it is lying on its back. Avoid
over-stretching the ventral skin.
2. Make a midline incision in the abdominal skin with small scissors, leaving the
underlying muscles uncut. Continue this skin incision up to the jaw and back to the
urogenital opening(s). If the specimen is a male, cut the skin around both sides of
the base of the penis and continue the cut back to the anus.
3. Pulling with both hands in opposition, strip the skin from the underlying body wall.
There is a group of muscles and blood vessels going to the skin under the arms,
which you may have to cut. Pin the skin out as far as it will go. Adjust the position
of the pins through the legs so that the knees are fully extended and spread out.
4. Identify the ribs, attached at the midventral line to the sternum, which encloses the
thorax. The intercostal muscles between the ribs provide much of the pumping
force for lung respiration and the whole thoracic wall is much more resilient and
springy than the thin abdominal wall. This is so thin that the abdominal viscera can
easily be seen through the muscle layers.
5. If the specimen is female the mammary glands will be visible, lying between the skin
and the body wall.
Abdomen:
1. Pinch up the abdominal wall with forceps and snip through the muscle layers. Make
a median incision up to the end of the sternum and then cut transversely along the
junction between thorax and abdomen. Pin the abdominal wall out sideways to
display the viscera. Identify the dark red lobes of the liver almost covering the
stomach. The small intestine is tightly coiled and held together by a thin sheet of
connective tissue called mesentery. This is actually a double layer continuous with
the abdominal cavity lining, which is called the peritoneum. The mesentery supports
the weight of all the viscera like a hammock. Loops of the thicker, darker large
intestine or colon may also be seen at this stage leading down to the rectum. The
external opening of the rectum is the anus.
2. The rectum stores solid indigestible wastes; liquid wastes containing waste products
of metabolism are stored in the urinary bladder. In your specimen this may be full
and easy to identify, but when it is empty, the bladder is small and not easily visible.
Close to the stomach is a small, dark red, tongue-like organ called the spleen. The
spleen has no connection with the digestive system, but is an important adjunct to
the circulatory system. Depending upon the age and nutritional history of your rat,
you may find yellow fat bodies scattered throughout the abdomen.
Thorax:
1. Projecting from the posterior end of the sternum is a cartilaginous flap. Grasp this
with forceps and pull it up to expose the diaphragm. This muscular sheet acts as
the floor of the airtight thoracic cavity and comprises an important component of the
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respiratory pump. The pressure inside is usually less than that of the atmosphere;
snip a small hole in the side wall of the thorax and observe the air rushing in. The
lung was previously pressed closely against the diaphragm due to this negative
pressure, now it will spring away from the diaphragm exposing the pleural cavity.
Repeat this procedure on the other side.
2. Insert the scissor blade again into the slit already made and cut through the thoracic
wall close to the diaphragm, but leaving the diaphragm intact. Make lateral cuts
right through the ribs so that a triangular piece of the thoracic wall can be removed.
The apex of the triangle should just cut into the collar bone or clavicle.
3. Examine the diaphragm carefully. Notice its natural dome shape. All the muscle
fibres are arranged radially around the edge so that when they contract they pull the
central connective portion down flat.
4. Look for the windpipe or trachea. It is easily recognizable by the rings of cartilage
strengthening the walls give it a cross-striped appearance. The trachea then divides
to form two bronchi, not visible here because they are surrounded by lung tissue.
Branches of the bronchi are termed bronchioles, and these terminate in thin-walled
sacs called alveoli. These sacs give the lungs a spongy nature. Note the division of
the lungs into several lobes.
5. Nestled between the lungs is the four-chambered heart. Two small sacs, the atria,
are found on either side of the anterior portion of the heart. The larger, posterior
portion of the heart consists of two ventricles, divided internally by a septum.
From the left ventricle emerges the arch of the aorta, a thick-walled artery whose
branches carry blood to all regions of the body.
Digestive system:
1. Identify the esophagus running down the neck beside the trachea, through the
diaphragm and opening into the stomach.
2. The stomach is enfolded by the liver lobes but the liver itself is connected to
the rest of the digestive system only by the hepatic veins. Use pins to hold the
liver lobes away from the stomach (put the pins against the organs, not through
them).
3. Bile, manufactured by the liver from the hemoglobin of old red blood cells, is
carried by the bile duct into the initial part of the small intestine that is known as
the duodenum. In many mammals the bile duct is joined by another duct
leading from the pancreas. This small duct leads from a rather diffuse pancreas
lying between the loops of the small intestine in the mesentery.
4. At the junction of the small and large intestines is a caecum ending in the
appendix (relatively much larger than in humans).
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5. The initial part of the large intestine is called the colon. The last part of the
digestive tract is the rectum where fecal material is formed into pellets and
stored before elimination through the anus.