Like last week's plant evolution lab, this session involves careful
observation of many specimens and demonstrations including slides as
well as live animals to show the incredible diversity in the Kingdom
Animalia. Although the animals you will see appear very different,
they are all constructed according to a few critical design requirements
that reflect a handful of important and innovative structures like
shells, skeletons, and body cavities.
Based on these simple variations of and additions to a few original body
plans, you should be able to tell something about the relationships and
classification of various invertebrate phyla. You will then use
these characters in a completed chart to construct a simple dichotomous
key that can be used to identify all the animals you will see in lab.
Read this entire lab before beginning on the work to be done.
• To learn the
major characteristics of animals, and be able to identify all lab
specimens to at
the phylum level.
• To become
familiar with major advances in the evolution of invertebrates.
• To develop a
dichotomous key that can be used to identify animals in eight phyla.
• To understand the
history, relationships, and life cycle of animals.
• To practice
observation, note-taking, and drawing skills.
The Kingdom Animalia consists of organisms with the following
1. Eukaryotic cells
2. Multicellular organization
3. Motility using muscular contraction
4. Feeding by ingestion
Although the fossil record of animal life goes back more than half a
billion years, early traces are ambiguous and may not be animals at all,
or may be animal life of a kind totally unlike what we see today.
Modern animal life dates approximately from the beginning of the
Cambrian Period, around 590,000,000 years ago. By the middle of
this geological period some 40 million years later, most of the animal
phyla still in existence today had evolved their basic forms and become
diverse. Each phylum shows a body plan that is characteristic or
fundamental for all its members, sometimes called a bauplan,
German for "basic design". The complexity of this plan varies
among the phyla, and those with more complex plans are usually thought
to be more highly evolved than those with simpler plans. Though
this is not a hard and fast rule, parasitic members of phyla that are
otherwise complex can become very simplified as a result of their
habits. This laboratory will survey some of the major features of
animal body plans.
will review with you the six major features of animal body plans that
are discussed here.
This refers to the general appearance and arrangement of body parts.
While some animals, such as sponges, have an amorphous appearance and
may not show any symmetry, most animals fall into one of two categories.
Bilaterally symmetrical animals can be divided into two halves
that are mirror images of each other only along one axis. Humans,
insects, worms and octopi are good examples of this type of symmetry.
Bilateral symmetry is associated with movement; such animals creep,
swim, run, or fly about in search of food and favorable environments.
On the other hand, radially symmetrical animals may be divided
into mirror-image halves along more than one axis, and often along
almost any diameter of the circle drawn around them. Examples are
sea anemones, jellyfish, starfish and sea urchins. Radial symmetry
is adaptive for animals that live their lives attached to something.
Being radially symmetrical makes it possible for them to detect stimuli
coming from all directions, and also to hunt food with equal facility
all around them.
The clear connection of symmetry with habit can be seen by looking at
the changes in the life of an individual starfish. The starfish
begins life as a bilateral larva. The larva briefly attaches to a
shell or rock and transforms into a radially symmetrical adult.
But adult starfish do not remain attached; they move about slowly!
Experiments have shown that starfish with 5 arms almost always move with
one specific arm extended ahead. Thus we could postulate that they
are on their way to becoming bilateral again the "front" arm could
become a head as a result of having abandoned their attached way of
This simply means the presence of a head. Cephalization occurs in
all bilaterally symmetrical animals because there is an advantage in
having the end that goes first, as the animal moves, fitted out with
sense organs—on the premise that it is better to know where you are
going than where you have been. Because a lot of computational
power is needed to deal with sensory stimuli, the nervous system tends
to be larger in the head as well. And finally, food can be
captured more effectively if the mouth is also at the front of the body.
3. Complete gut:
Strange as it may seem to us, many animals do not have a gut with two
openings—the same opening serves as both mouth and anus. Because
the cavity into which this "mouth" opens also serves as a rudimentary
circulatory system, it is called a gastrovascular cavity.
This arrangement occurs in flatworms (Platyhelminthes) and cnidarians (Cnidaria:
jellyfish, anemones, etc.). It would seem less efficient because
once the gastrovascular cavity is filled with food, the animal must wait
until it is all digested before expelling the wastes the same way the
food came in.
Having an anus, on the other hand, is a big plus. It means the
animal can eat more or less continuously, since there is one-way traffic
in the gut.
4. Body cavity:
As animals grew larger in body size, they faced a problem that occurs at
all levels of living things — the surface/volume ratio. Everything
that enters a cell or an organism has to come through a surface.
But as size goes up, volume increases much faster than surface area.
Volume grows as the third power (cubed) of length, while surface only as
the second power (squared), so as an animal's length increases from 2 to
4 units, its surface area increases from 4 to 16 square units and its
volume from 8 to 48 cubic units.
Relying on a simple, tubular gut to supply food would mean that the
animal would have to stay thin but grow longer and longer to get the
right surface/volume ratio. Long thin bodies are very fragile
though they certainly do exist in such animals as the horsehair worm,
which may be hundreds of times as long as wide. A better solution
might be to coil a long gut up inside a short body; other tubular organs
could also be packed in a smaller volume by coiling.
The coiling of tubular organs requires that the body be hollow.
Thus the formation of a body cavity, or coelom (pronounced
see-lum) represents an important advance. Technically, there are two
basic types of cavity, the so-called pseudocoelom and the
eucoelom, or "true coelom," but the distinctions between them are
technical and there is little evidence to suggest that one works better
than the other.
A fluid-filled coelom of some invertebrates provides other advantages.
The fluid, as it sloshes around, can circulate food and oxygen and pick
up wastes. Because a fluid cannot be compressed, if an animal
clamps down the muscles of its body wall on the coelomic fluid, it
becomes hard and rigid—the coelom can function as a hydrostatic
Many animals that swim or creep have no skeletons at all, not even a
hydrostatic one. A flatworm crawls by using tiny projections of
the cells of its underside, called cilia, against the surface. A
jellyfish swims by partially closing its bell against the resistance of
the water. Effective movement using limbs to walk or swim,
however, requires a skeleton, some hard structure for internal or
How a hydrostatic skeleton works in this system will be explained in the
next section. Right now we can recognize two kinds of skeletons by
their positions— endoskeletons are like ours, and are on the
inside of the body forming a structural framework, while exoskeletons
are on the outside and look like a suit of armor.
Endoskeletons occur primarily in chordates, where they are composed of
living cartilage and bone, and in echinoderms, starfish and their
relatives, where they are made of massive crystals of calcite.
Exoskeletons are found in arthropods. They are not living and are
secreted by the outermost layer of cells. Some exoskeletons
contain minerals, primarily calcite, but most are made of tough proteins
and a sugar polymer, chitin.
Which is best, an endoskeleton or an exoskeleton? There are
advantages to both. An exoskeleton doubles as a protective layer
and as waterproofing. However, because an exoskeleton is not
alive, if the animal is to grow, it must be periodically shed and
re-secreted. During this time an arthropod may be immobile, soft,
and very vulnerable. Chitin and protein are also not terribly
strong, so most animals with chitinous exoskeletons are limited to small
sizes. Bone, on the other hand, is alive and can grow. It is
strengthened by mineral deposits, primarily calcium phosphate, and is
quite strong. So it is no surprise that chordates number amongst
them the largest animals that ever lived. And also because it is
alive, an endoskeleton of bone can repair itself.
Distinguishing between a skeleton and a shell can be a problem.
Generally, a shell is only for protection and does not provide either a
structural framework or a system of levers which muscles operate as does
the skeleton. The most obvious and important group of animals with
shells are the molluscs—clams and oysters, snails, and squids and
octopi. In the latter two groups, the confusion typical of
biological systems comes in: some squids have brought their shell
to the interior of the body, and it really does act as a skeleton!
A segmented animal has a body that consists of repeating identical or
similar units, called segments. The division of a
fundamentally worm-like, bilateral body into segments may be thought of
as the final major adaptation. This major characteristic occurs
only in three of the 35 or so animal phyla. Therefore it is likely
that segmentation only arose two or three times in the evolutionary past
In the chordates, segmentation arose as an adaptation to efficient
swimming, and is primarily a characteristic of the muscles,
skeleton, and nervous system. The muscles are arranged in V-shaped
bands, and as a wave of contraction passes down them, alternating on
each side, the body sweeps into graceful curves which push against the
water and propel it forward. Quite naturally, the backbone and
nervous system followed this plan of muscular units.
In the annelid worms (such as earthworms) segmentation arose as an
adaptation to burrowing in soft mud. The coelom
(hydrostatic skeleton) was divided into compartments. Because
liquids cannot be compressed, the volume of each compartment must remain
the same. Each compartment is wrapped in two layers of muscles,
circular and longitudinal. When the longitudinal muscles are
contracted, they squeeze the segment into a broader, shorter form that
can anchor a worm in a burrow. The contraction of the circular
muscles makes the segment longer and thinner, pushing the whole animal
ahead of it. By alternating these two shapes, the worm can anchor
itself in the soil or mud or get a push to move further into the
substrate. The muscles require nerves to operate them, so these
are segmentally arranged, too. Since each compartment must be
sealed, it must have its own set of certain organs. Material is
conducted from compartment to compartment by a circulatory system of
Animals whose segments are all alike show homonomous segmentation
such as the earthworm. But in many advanced animals, the segments
have become specialized for different functions — heteronomous
segmentation. The segments may also be arranged in groups
called tagmata (singular tagma). For example, in
both chordates and arthropods, the segments nearest the front have
become specialized for feeding, breathing, and gathering sensory
information. They have often fused with the head; most arthropods
have a head that includes at least 6 segments. Your head includes
10 segments, marked in your nervous system by 10 pairs of cranial (head)
nerves. For example in Chordates, heteronomous segmentation can be
seen in the spinal column and ribcage.
Arthropods have armored their segments with exoskeletal plates. The
insects show the greatest level of heteronomous segmentation and
tagmatization; each insect body consists of three multisegmented units.
The head collects information and feeds, the thorax is
concerned with movement, walking and flying, and the abdomen
contains most of the internal organs, including those involved in
From this listing of body plan characteristics, you can see that the
simplest animals have the smallest number of these features, and the
most complex the largest number. Since the characteristics were
probably acquired by animals in the order in which they are listed, they
can also be used to show the course of evolution, as in the
phylogenetic tree attached to this lab.
In lab today you will examine a variety of animals to determine their
body plan features, and learn the names and characteristics of some of
the major animal phyla.
DEVELOPMENT IN ANIMALS:
The similarities in early development among all animals can be seen by
examining the slide of starfish embryos going through the early stages
of this process. Observing these early stages will help you to
understand several of the features of the body plan that have been
presented—bilateral symmetry, cephalization, gut, and coelom
Obtain a slide of starfish development. Locate the cells with the
clear nucleus, which contains another structure called a
nucleolus. This is the unfertilized egg of the
starfish. Now find single cells that are exactly the same size but
that have no visible nucleus. You are likely to see the
fertilization membrane around such cells. The fertilization
membrane looks like a cellophane wrap around the cell. This is the
fertilized egg or zygote. If you cannot find a
fertilization membrane here, you might see it more clearly as you move
to the next stage of development. Draw and label (the items in
bold) the unfertilized egg and the zygote on the sheet provided.
Estimate the size of the zygote in micrometers. If you
can’t remember how to do this, consult the directions for the first
laboratory in this manual. Keep your measurement in mind as you
observe later stages of starfish development. Do the embryos get
larger, smaller, or stay the same size as they continue to develop?
Be prepared to explain your observations.
The zygote divides to form the two-cell stage. Cell
division in embryos is referred to as cleavage. The cells
are called blastomeres. Draw the 2-cell stage, labeling
the fertilization membrane.
Find an eight-cell embryo and draw that. Notice that
even though the embryo contains more cells, blastomeres, that the
overall size of the embryo is essentially the same as the zygote.
The next stage to look for is the morula, 16 to 32 cells in a
tight ball. Draw an embryo at this stage.
Find an embryo that appears as a hollow ball of cells. The space
in the center of the embryo is the blastocoel. An embryo at
this stage is referred to as a blastula. Sketch the
After the formation of the blastula, one surface of the hollow ball
begins to move into the space. With this inward sweep of cells,
two tissue layers are established: the outer surface layer called
the ectoderm and the inner layer called the endoderm.
The opening is called the blastopore and leads to the primitive
gut tube, or the archenteron. Find an embryo at this stage, called
the gastrula, and draw and label the structures:
blastocoel, gut or archenteron (also called the gastrocoel), endoderm,
ectoderm, blastopore, and mesoderm.
In Chordates and Echinoderms, like you and this starfish, respectively,
the blastopore becomes the anus of the primitive gut tube, and this tube
touches the other surface of the embryo and forms the mouth.
Organisms in this line of descent, with the two openings formed in this
order, are called the deuterostomes. The other phyla are
called protostomes; in protostomes the blastopore becomes the
mouth. In both of these lines of descent, a space, called a
coelom, may form within a third layer of tissue called the mesoderm
which arises between the outer surface of the body, the ectoderm, and
the gut, composed of endoderm. The three primary tissue layers,
endoderm, ectoderm, and mesoderm, differentiate into the following
structures in the mature organism:
endoderm: digestive organs and lungs
mesoderm: muscles, connective tissues, skeleton
ectoderm: skin, brain, nerves.
The next developmental stages occur relatively rapidly. You should
be able to find a multicellular larval form, that is no longer
spherical, that possesses the third tissue layer, the mesoderm,
which cannot be specifically detected but makes up some of the internal
structures of the organism. Draw the larva.
This background on development, the diagram provided in this lab, your
textbook (remember to use its index!), the specimens and information
available in the laboratory, as well as the questions and observations
concerning each phylum included here, will help you complete the chart
for the major phyla.
SURVEY OF THE
Examine preserved specimens of jellyfish and sea anemones, both members
of the Phylum Cnidaria. What kind of symmetry do they have?
Is a complete gut present? Do they have a skeleton?
Name several characteristic specimens and fill in the column under
Cnidaria in the chart.
Obtain a live flatworm, or planaria of the Phylum Platyhelminthes,
in a depression slide and watch it under a microscope. How does it
move? Is it segmented? Draw a sketch of the specimen.
Observe the other platyhelminthes on display—flukes and tapeworms.
Fill in the proper column in the chart.
Make a wet-mount slide from the culture of vinegar eels, a minute
nematode worm from the Phylum Nematoda. How do the worms
move? This worm has a pseudocoelom that helps it move in this
Observe the other "roundworms" on display. Fill in the proper
column on the chart.
Observe a live earthworm from the Phylum Annelida in the
fingerbowl. Notice how the shapes of the segments change as the
worm crawls across the towels. Does this worm show homonomous or
Observe the segmented worms on display. These worms have a true
coelom. Fill in the proper column on the chart.
Look at the trays of insects, Phylum Arthropoda and other
specimens provided; identify the three tagmata. Is this homonomous
or heteronomous segmentation? What is the nature of the skeleton?
How many tagmata does a crawfish or lobster have? Observe other
arthropods on display. Fill in the chart.
A display of shells on demonstration suggests something of the variety
of types to be found in the Phylum Mollusca. Make careful
observations concerning the other mollusks available. Refer back
to the information given earlier in this lab concerning skeletons v.
shells. Remember, a shell is not a skeleton.
examples of a few of these shell types and other specimens of interest.
A small sea animal called a lancelet, Amphioxus or
Brachiostoma, will be our example of the Phylum Chordata.
Remember that YOU are an example as well. Look at the whole
specimen mounted on a slide and note the form of the segmentation.
A stiff rod runs the length of the body; this is the notochord.
Its elasticity opposes the contractions of the muscles. Notice
that some of the anterior segments bear gill slits. These
segments become part of the head in the vertebrates, which probably
descended from an ancestor much like the lancelet.
Review the other chordate specimens on display. Fill in the
characteristics on the chart.
The last phylum to be examined is also the most peculiar, the Phylum
Echinodermata. A starfish is a typical example. What
kind of symmetry does it have?
Examine other specimens available in this group. Fill in the chart
for this phylum.
Your final activity today is to make up a dichotomous key, similar to
the one you used several weeks ago, that could be used to identify the
eight animal phyla you have just studied. Hand the key in as a
part of your evaluation this week.
2. Study for
your final practical which includes labs 7-11.