IB 201: LABORATORY SESSION 13: ANIMAL DIVERSITY
Goals: Introduce students to the diversity of animal. Relate aspects of the biology of
various animal phyla to their evolutionary relationships. Consider the variation within
and among animal and use the variation to address the issue of constraints in evolution.
Assignment.
1) Identify the live specimens in the lab to phylum and provide visible characteristics for
each.
2) Map animal structure onto a phylogeny.
3) Write a short assignment that relates the diversity of animal structure and biology to
constraints in evolution. What does it mean to say that evolution is constrained?
Introduction
Three labs ago we did a survey of the major evolutionary groups of plants. This week we
will repeat the process for another major group of multi-cellular organisms – the animals.
We have no animal equivalent of the greenhouse so we will be using living and preserved
specimens in the lab.
Animals are by far the most diverse group of living organisms in terms of described
species with well over one million species that have been named by scientists. Animal
diversity is very well known relative to most other groups of organisms. There are two
reasons for this. Human beings are animals and it is natural that we have taken more of
an interest in the organisms that are most similar to ourselves. Also, and of more
relevance to the study of animal evolution, animals have a much greater diversity of body
form than any other group of organisms. For centuries biologists have been studying the
details of development and composition of animal bodies in an attempt to work out the
evolutionary relationships among different animal phyla (a phylum is a major group of
animals such the arthropoda or the mollusca). The study of the evolution of animal body
form has led to debates about the nature of evolution itself.
Specifically there has been considerable controversy about the role of animal body plans
in constraining evolution. Virtually all animal phyla with substantial fossil records first
appear very early in the history of animals. Some evolutionary biologists have
interpreted this pattern as evidence that, once basic body plans were established, it
became very difficult, if not impossible, to evolve a new body plan from pre-existing
body plan. In other words, evolution was constrained so that animals could evolve
variations on a theme but not develop an entirely new forms. Other biologists have not
been convinced by this argument. They argue that there is no direct evidence for
constraints and that experimental studies of evolution do not support the idea that body
plans cannot be changed by the appropriate selective pressure.
In the following two sections we review some of the terminology used in describing
animal structure and an over-view of some of the more important animal phyla. Students
who are also in IB 202 should find some this material familiar although the context in
which it is presented is somewhat different.
Terminology
Bilateral vs. Radial Symmetry – Unlike plants most animal bodies have well-defined
overall shape. Most animals are bilaterally symmetrical – the body has a single plane
which divides the body into two equal halves. Think of the human body. It can be
divided into equal left and right halves but not equal top and bottom halves or back and
front halves. Some animals such jellyfish or starfish have radial symmetry in which the
body can be divided into equal halves in multiple planes. For an analogy, think of a pie
or a cake – they can be cut in half many different ways. A few animals, such as many
sponges, are asymmetrical – there is no plane that can divide them in two.
Radial Symmetry
Bilaterial Symmetry
Coelom – The coelom is the body cavity, the space in which many of the organs are
found. A typical animal can be thought of as a hollow tube containing the various organ
systems. The digestive system then forms a tube within the coelom (tube inside a tube
body form). Some animals lack coeloms (they are acoelomate). Among animals with
coleoms there is variation is how the coelom is formed (see below).
Blastopore – During the develop of animal embryos the zygote undergoes mitosis many
times to form a ball of cells (blastula). The blastula then undergoes a process known as
gastrulation. During gastrulation a pocket forms in the blastula, which leads to the
formation of a hollow ball of cells with an opening. This opening is called a blastopore,
the ball of cells is called the gastrula. The empty space inside the ball of cells becomes
the coelom.
Blastula
Gastrulation
Gastrula with blastopore
Deuterostome – An animal in which the blastopore forms the anus and the mouth is
formed later.
Protostome – An animal in the blastopore form the mouth and the anus is formed later.
Pseudocoelom – Some animals have a body cavity that forms in a different way. Instead
of forming from the interior of the gastrula, it forms later, once different layers of tissue
have formed.
Organ Systems – Animals have evolved a series of specialized structures to perform
specific functions. These structures are called organs. Organs are organized into organ
systems. The major organ systems of animals are: nervous system, respiratory system,
circulatory system, digestive system, excretory system, muscular, skeletal, and
reproductive systems.
Segementation – Many animals are segmented. Their bodies are divided into a series of
distinct sections. An earthworm is an obvious and familiar example.
Lophophore –The lophophore is a specialized feeding structure (a ring of cilia
surrounding the mouth) found in a few minor phyla including the Bryozoa. The
lophophore is used in fllter feeding.
Animal Ways of Life.
Habitats – Animals live in all major habitats in earth, in the ocean, fresh water, in the soil,
on land, and inside other organisms (parasites).
Feeding – Animals feed in a variety of ways. Herbivores eat plant material and
carnivores eat animal material. This can include solid food but also liquid food as well as
many animals feed on the body fluids of plants and or animals. An important way of life
for many marine animals is filter feeding. Many animals filter small organisms and
pieces of organic material from the water that passes over their bodies. Other animals are
parasites and live inside of other organisms. They generally have greatly reduced
digestive systems.
Reproduction and Development – Most animals are capable of sexually reproduction but
many are also capable of asexual reproduction (e.g. budding in sea anemones). Animals
with sexual reproduction can be hermaphrodites (individuals are both male and female at
the same time) or dioecious (have separate sexes). Many animals have larval forms that
are radically different from the adults in both form and ecology. Marine animals often
have larvae that lead a planktonic existence and can disperse vast distances.
An Overview of Animal Phyla
The following is brief over-view of the major animal phyla as well as a few of the minor
phyla. Please refer to the Tree of Life web page, Invertebrate Biology text, or other
resource (see bibliography) for more details.
The first three phyla are acoelomate animals. They lack a body cavity. Both the Cnidaria
and the Platyhelminthes have organs but the sponges lack most of the structures found in
other animals
Porifera. These are the sponges. Sponges are mostly marine (found in the ocean) with
some species in fresh water. Sponges like many marine animals are filter feeders. They
eat by picking small particles of food out of the water as it passes over or through them.
Sponges are the simplest of the major groups of animals, lacking organs and organ
systems. Water is pumped through the hollow interior of a sponge by the action of many
individual cells. Food is extracted from the water by individual cells as well.
Cnidaria. This group includes corals, sea anemones, jellyfish, hydras, and other, less
familiar organisms. Most are found in salt water with a smaller number occurring in
freshwater environments. All of these animals have radial symmetry. They have a gut
with only a single opening (no separate mouth and anus) and a nervous system with no
central control. Like sponges, Cnidarians capture food from the water as it passes near
them. Unlike sponges, the food they eat can be quite substantial, including small fishes.
Cnidarians capture larger prey items by injecting them with toxins. In a few species these
toxins can be extremely painful and, much more rarely, even fatal to human beings.
Platyhelminthes. These are the flatworms, including tapeworms and flukes. These
animals include both parasites and free-living species. Free living species are mostly
found in both salt and fresh water although some are found in very moist habitats on
land. Unlike the two phyla above the Platyhelminthes have a central nervous system, a
digestive system with a mouth and an anus, and are bilaterally symmetrical.
The next phylum, the Nematoda, contains animals with a psuedocoelom. The
development is protostomate.
Nematoda. These are the roundworms. The Nematodes may be the most
underestimated group of animals on earth. They occur in almost every environment on
earth but because they are mostly small and inconspicuous, they are usually only noticed
by humans when they are an economic or health problem. Nematodes are small slender
animals generally with a body that is round in cross section without many external
features and tapered at both ends. There is no segmentation. Nematodes may be
parasites, predators, scavengers, or plant eaters. Many species are serious plant pests.
The next five animals are protostomes with a ‘true coelom’
Annelida. These are the segmented worms, including earthworms, leeches, and
polychaete worms. Annelids have segmented bodies, meaning that the body consists of
series of similar segments that are arranged in a line. Leeches, earthworms, and their
relatives are found in freshwater and in moist environments on land. Polychaetes are
found in the ocean.
Mollusca. These are the mollusks, including clams, snails, octopi and their relatives.
This is a very large and well known phylum. In terms of named species (species that
scientists have described and given names to) this is the second largest in the world.
Most mollusks have a shell, composed mostly of calcium. The shell may be a single
piece, as in the snails and some other mollusks, or it may be two pieces joined at a hinge,
as in the clams and their relatives. Octopi and their relatives have lost most or all of their
shells and are much more mobile than other mollusks. Mollusks are not segmented and
have a reduced coelom.
Arthropoda. These are the arthropods, including insects, crustaceans, centipedes,
millipedes, spiders, scorpions, mites and ticks, and their relatives. The arthropods have a
segmented body with an exoskeleton composed largely of a substance named chitin.
Arthropods have legs with joints. About 80% of the known animal species are
arthropods (mostly insects) and there are many more species that have not been named.
Arthropods are found in almost every environment on earth and are one of only two
groups of animals on earth that have been very successful at living on land outside of
moist environments.
Bryozoa – A common group of marine organisms. The Bryozoa are often overlooked
are mistaken for sponges or corals. Individual bryozoans are tiny filter-feeders that form
colonies. The feeding mechanism of a Bryozoan is called a lophophore, a structure found
in a number of other animal phyla.
Sipuncula. These are the peanut worms. A small group of marine organisms that
primarily filter feeders. Sipuncula are unsegmented and wormlike. They lack circulatory
and respiratory systems and often live in the sand or mud. Details of their development
are similar to those of mollusks and annelids.
The remaining two phyla are deuterostomes.
Echinodermata. These are the echinoderms, including sea stars (starfish), sea urchins,
sea cucumbers, sand dollars, brittle stars, and their relatives. The echinoderms have
radial symmetry and are only found in saltwater habitats. They can be predators, plant
eaters, and filter feeders. Most species move around, very slowly, on a large number of
very small tube feet. Echinodermata have radial symmetry and a hydrostatic (waterbased) skeleton.
Chordata. These are the vertebrates and a few other small marine organisms.
Vertebrates (mammals, birds, fishes, reptiles, amphibians) are familiar to most people so
they will not be described in detail. Chordates have a notochord (stiff structure running
the length of the body – the basis of the spinal column in vertebrates) and a dorsal never
cord.
Hypotheses About the Evolutionary Relationships of Animals.
The traditional hypothesis for the evolution of animal body plans has been as follows
Porifera Cnidaria
Platyhelminthes
Protostomes (true coelom)
Deuterostomes
Next tree
starts here
Under this model the position of animals with a pseudocoelom is unclear.
Modern molecular techniques have revised the tree somewhat. The basal portion of the
of the tree is the same. However some unexpected relationships among the protosomes
and pseudocoelomates have been discovered as shown below. Mollusks and annelids,
once thought to be closely related to arthropods are now thought to be the close relatives
of many of the minor phyla with lophophores while arthropods are now thought to be
closely related to annelids.
Sipuncula Bryozoa Mollusca Annelida Arthropoda Nematoda
Deuterostomes
B
A
The clade marked A on the phylogeny above is referred to as the Lophotrochozoa. This
name refers to the lophophore that is found in the Bryozoa and some other phyla not
included on this tree. The rest of the name refers to a type of larva shared by many of the
phyla in the group. The clade marked B of the phylogeny is called the Ecdysozoans.
This name refers to the fact that all of these organisms growth by molting – they shed
their cuticle or exoskeleton periodically in order to grow larger.
Exercise I: Animal ‘Scavenger Hunt’
A variety of living animals from different animal phyla can be found in the lab and in the
aquaria in the adjacent room (room 313). Your TA will assign you six live animals.
Look at the animals under the microscope (if appropriate) and using this lab and the
material available in lab please fill out the following table and present it to your TA
before moving onto the next exercise. Provide at least one identifying characteristic per
phylum and a total of ten.
Animal (Common Name)
Phylum
Exercise II: Animal Structure and Evolutionary History.
Visible Characteristics
Once you have identified your living animals your TA will assign you to a phylum. Your
assignment is to document variation in structure and way of life in that phylum. The
table at the end of the lab is designed to guide you in doing this. Using the material
available in the lab complete the following table at the end of the lab (you do not have to
fill in every row in the table). Once you are finished present the data to your TA and put
it up on the board. Make sure you record data on all of the phyla before leaving class.
Exercise III: Constraints on Evolution.
Using the data gathered by class write a one paragraph (maximum) document about
constraints in the evolution of animal phyla. What do you think it means to be
constrained? Do you think that animal phyla are constrained in their evolution. Your
answer does not have to be a strict yes or no – you can can answer both yes and no
(depending on what constraint means and on particular circumstances). Plot whatever
information you deem necessary onto one or more phylogenies to support your case. The
information should come from exercise II and the phylogenies should be based on the
phylogenetic information provided in this lab.
Phylum ________________________
Feeding Modes (filtering, etc.)
Structures Used
Methods of Locomotion
Structures Used
Skeletal Structures
Way of Life
Life Cycle (Sexual/Asexual/Larval/Direct
Development)
Environment(s)
Sensory Structures
Way of Life