Vertebrate Zoology – Lab 2
Structural Changes in Basal Chordates and Vertebrates
In lecture, we have discussed some of the basic structural changes associated with the evolution of
early, basal chordates and vertebrates. In this week’s lab, you will have the opportunity to observe
preserved specimens of four major extant groups within the vertebrate lineage that illustrate
symplesiomorphies associated with the primitive chordate condition as well as some of the
apomorphies that allowed for the progressive diversification of vertebrates. By following the sequence
of new and modified characters that emerge in each of the four groups, you should gain some
appreciation for the increased range of possibilities in resource utilization, habitat specialization and
behavioral complexity that become available with these early evolutionary adaptations.
Cephalochordata
The cephalachordates are considered by most zoologists to be the closest extant relative (CER) to the
vertebrates. There are 30 species of cephalochordate, most in the genus Branchiostoma (formerly
know as Amphioxus) and they are commonly referred to as lancelets. Although they spend most of their
time with the posterior portion of their body buried in the sand in warm marine environments, they
have the capability to move through the water column via lateral body movements. Your slide box
contains both a whole mount and cross sections through portions of the body. Using these slides,
identify the features in bold and consider the associated questions. Although I have included figures in
this handout, I would strongly encourage you to use the blank spaces provided to make your own
sketches or drawings of what you observe. In many instances, the staining techniques used to bring out
the contrast between differing tissue types creates an image that may differ significantly from what you
observe in the line drawings I have provided. Since you will be working with stained specimens on the
practical exam, it may be advantageous to have these hand-drawn figures as a study aid.
The most anterior portion of a cephalchordate is known as the rostrum. Just posterior to tip of the
rostrum in the dorsal region are two structures that extend the entire length of the specimen, the nerve
cord and ventrally, the notochord. In the most anterior portion of the body lies the vestibule,
containing the buccal cirri and the wheel organ. These two structures work together to bring water and
food particles into the oral region. Posterior to these structures, you will find the pharynx. Food
particles pass into this region and are trapped along the mucous covered pharyngeal bars and by way of
ciliary action, carried to the epibranchial groove along the dorsal surface of the pharynx. Water passes
out through the pharyngeal slits and the food particles are carried along the epibranchial groove into
the intestine. Ventral to the pharynx is the endostyle, the tissue that produces the mucous. On either
side of the pharynx and notochord, you should be able to see the myomeres, separated by myosepta.
Contractions of the myomeres provide the lateral body movements used by cephalochordates for
propulsion through the water column.
After identifying these features, consider the following questions:
1. Does filter-feeding require any specialized sensory capabilities?
2. Is there any evidence of specialization or organization of the neural tissues you observed?
3. Given the size of the whole mounts you observed, how would cephalochordates achieve gas
exchange to obtain oxygen and eliminate carbon dioxide?
Use the space below to create any figures or drawings you may need to aid in your study for the
practical.
Petromyzontidae
The next set of slides and specimens you will work with are lampreys, representative of the extant
jawless fishes or Agnathans. You will be looking at slides of the larval form of lampreys, known as an
ammocoetes, mounts of the cartilaginous skeleton of the adult lamprey and plastimounts of a crosssection through the anterior portion of an adult lamprey.
Lampreys have a bi-phasic life cycle consisting of a non-reproductive, filtering feeding ammocoetes
larval form and a reproductive, often parasitic, adult form. Like the cephalochordate, larval lampreys
remain buried in soft sediments with their anterior portion exposed. Using the muscles in the pharynx,
they pull water in through the oral hood and force it out through the gill slits. In addition to supporting
the tissues that create the gill openings, the branchial cartilages also support the gill lamellae in the
pharyngeal region. You should be able to see a fairly large structure dorsal to and slightly posterior of
the oral opening. This is the brain, which transitions posterior into the spinal cord. The notochord lies
ventral to the spinal cord. Layers of muscle, separated by myomeres, lie on either side of the pharynx
and as you look posterior, also lateral to the notochord, spinal cord and intestine. Depending on the
quality of the slide preparation, you may also be able to identify elements of the circulatory system,
including the heart, and dorsal aorta.
The shift from filter-feeding larvae to parasitic adult entails changes in the structure of the oral region in
lampreys. In the adult lamprey cross-section, you should notice the keratinized teeth and annular
cartilage surrounding the oral opening. These structures allow the lamprey to firmly attach to the side
of a host fish and, using the rasping surfaces on the muscular tongue, create an opening in the fish’s
body wall. Flesh from the host then passes through the pharynx and into the intestine. Again, note the
location of the brain, spinal cord, notochord, gill openings and gill lamellae. In both the cross-section
and mount of the cartilaginous skeleton, you should observe the branchial basket cartilages and the
elements of the chondrocranium, including the optic cartilage, otic capsule, neural arch elements and
the annular cartilage.
After observing these structures in the larval and adult lamprey, consider the following questions:
1. How do the derived characters you observed in lampreys differ from those in the lancelet? How
are they similar?
2. What are the major changes in the pharyngeal region?
3. What are the major changes in the neural structure?
4. What are the major changes in the skeletomuscular system?
5. What is the functional significance of any changes you observed when you compare lancelets
and lampreys?
You can use the next page to create any figures or drawings to aid in your study…
Chondrichthyes (Cartilaginous fishes)
Chondrichthyes are the first group of extant jawed fishes (Gnathostomes). On the whole mounts
available, observe the paired fins (pectoral and pelvic) and unpaired fins (dorsal, anal and caudal). Also
identify the spiracle and gill slits. On the dried chondrocranium, identify the rostrum, otic capsule,
olfactory capsule, postotic process, occipital condyle, foramen magnum and the individual foramina on
the ventral surface. On the whole mount of the skeleton, identify the structures associated with jaws,
including the palatoquadrate, Meckel’s cartilage, hyomandibular, and ceratohyal. The latter two form
the hyoid arch. Posterior to these structures are a series of five branchial arches. Note the structure of
the vertebrae and the position and structure of the pectoral and pelvic girdles.
Consider the following:
1. How has the structure of the pharyngeal region changed?
2. What are the similarities and differences in the structure of the supporting elements in the
pharynx?
3. Do you see any similarities in the structure of the jaws, the hyoid arch and the branchial arches?
Use the space below to create any diagrams or drawings…
Osteichthyes (Bony fishes)
The final group we will consider is the bony fishes. You may have noted that all of the elements of
Chondrichthyes were cartilaginous, with the notable exception of the teeth. As their name suggest bony
fishes possess true bone that is both endochondral and dermal in origin. On the skeletal mounts
observe the paired and unpaired fins and the vertebral elements, including the neural arch, neural
spine, hemal spine, centrum and basapophysis. On the skulls, identify the opercle, frontal and parietal
bones. Consider the structure of the elements of the jaw and how they differ from what you observed
in the cartilaginous fishes.
1. Given the position of the centrum and neural arch, what elements of the agnathans and
cephalochordates are included? What elements represent new structures?
2. What are the similarities and differences in the structure of the cranial skeleton of the bony
fishes compared to the cartilaginous fishes?