Animal Form and Function by Dana Krempels
Animalia is characterized by a distinct progression of complexity in form and function. Early
in animal evolution, body symmetry, embryonic germ layers, and ontogenetic origins
of major anatomical structures diverge as taxa branch from common ancestors.
Before you begin this workshop, be sure you are able to
1. List synapomorphies that distinguish animals from all other eukaryotes
2. Understand the meanings of asymmetry, radial and bilateral symmetries
3. Be able to recognize the major animal phyla on the basis of
a. body symmetry
b. embryonic germ layers (ectoderm, endoderm, mesoderm)
c. presence or absence of an internal body cavity
d. ontogeny and morphology of the internal body cavity
e. ontogenetic differences between protostomes and deuterostomes.
4. Be able to recognize acoelomate, pseudocoelomate and coelomate body plans
5. Distinguish between
a. spiral and radial cleavage
b. determinate and indeterminate cleavage
c. schizocoely and enterocoely
To review the items above, go to
http://www.bio.miami.edu/dana/160/160S16_15.html
A. Ontogenetic and morphological characters present in major animal phyla
Note the characters in the table below. Each should be placed on the phylogenetic tree (on the
next page) to indicate where it appears in the animals above that character. (Enter only the
letters, since there’s no room to write the entire character description.)
a. lophophore feeding apparatus
b. mesoderm lines parietal side of body wall
c. body cavity contains acellular mesogloea
d. coelom formed via enterocoely
e. mesoderm derived from endoderm
f. body cavity contains cellular mesenchyme
g. cellular division of labor
h. complete digestive system
i. diploblasty
j. triploblasty
Pronunciations:
cnidoblast – NEE’ - doh – blast
coelom – SEE’ – lom
enterocoely – EN’ - ter - oh - see - lee
lophophore – LOW’- fo-four
mesogloea – mes - oh – GLEE’ - uh
pseudocoelom – SUE’ - doh - see – lom
schizocoely – SKIZ’ – oh – see - lee
k. coelom formed via schizocoely
l. bilateral symmetry
m. radial symmetry
n. cnidoblast stinging cells
o. true tissues
p. nervous system embryonically dorsal
q. blastopore becomes the mouth
r. blastopore becomes the anus
s. trochophore larva
t. pseudocoelom a persistent blastocoel
1. What does the placement of the characters on the phylogenetic tree imply about evolutionary
relationships among the Bilateria (bi-luh-TIER’ - ee - uh)?
The evolutionary affinities of certain bilaterian phyla remain uncertain. In older phylogenies the Acoels
are considered a subtaxon within Platyhelminthes. It is now becoming clear that the Acoels are a basal
group that may resemble the very earliest Bilaterians: no body cavity, just the barest beginnings of true
tissues, and little more than a plate of skin with a very simple pre-gut.
Nematodes and other pseudocoelomates probably show the earliest method of body cavity formation,
lining the inside of the ectodermal wall (which will become the parietal wall of the adult body cavity) with
endodermally derived mesoderm. The pseudocoelom appears in various “unrelated” taxa throughout
Animalia, giving credence to the hypothesis that the earliest bilaterian body cavities were pseudocoeloms.
Schizocoely and Enterocoely are both more derived means of lining the body cavity with mesoderm.
These both result in a body cavity lined on both sides by mesoderm.
Note that the Deuterostomes’ “upside down” character states (relative to the protostomes) is likely a
derived condition, as it is found only in the Deuterostomes. All other animals (the vast majority,
including the most primitive forms) have protostome-like characteristics.
2. What does the placement of the characters on the phylogenetic tree imply about evolutionary
relationships among the Lophotrochozoa (low - fo - tro - ko - ZO’ – uh)?
The common larval form is highly suggestive of common ancestry. The presence of a lophophore
in only the Ectoprocta (bryozoans) and Brachiopoda (the Phoronida are not shown in this tree,
but they belong in this group, too) are monophyletic within the Lophotrochozoa, sharing the
derived lophophore found nowhere else in Animalia.
3. What does the placement of the characters on the phylogenetic tree imply about evolutionary
relationships among the Ecdysozoa (eck – dee – so – ZO’ – uh)?
Body cavity formation during ontogeny is evidently NOT a very good character for classification in this
group, as it contains both pseudocoelomates (Nematoda) and coelomates (Arthropoda). This is actually
further evidence that the pseudocoelom (a persistant blastocoel lined only on one side by mesoderm) is a
perfectly good, if primitive, way to make a body cavity.
4. Do a Google Image Search of the name each of the following taxa and view
photos/videos of them. What is the common name for animals in each of these phyla? For
each, indicate whether it is a protostome, deuterostome, or neither.
Taxon Name
Hexactinellida
Calcarea
Ctenophora
Acoela
Cestoda
Rotifera
Phoronida
Bivalvia
Hemichordata
Common name
Glass sponges
Calcareous sponges
Comb jellies
Acoel flatworms
Tapeworms
Wheel animalcules
Horseshoe worms
Bivalves (clams, mussels, etc)
Acorn worms
protostome/deuterostome/neither
neither
neither
neither
Protostome
Protostome
Protostome
protostome
protostome
deuterostome
Urochordata
Holothuroidea
Priapulida
Sea squirts
Sea cucumbers
Penis worms
Deuterostome
Deuterostomes
protostomes
4. Do another search. List some familiar examples of organisms in each of the following taxa:
Anthozoa (Cnidaria)
Sea anemones, corals, sea pansies, etc. LOOK AT PICTURES!!
Cestoda (Platyhelminthes)
Tapeworms (100% parasitic)
Cephalopoda (Mollusca)
Octopus, squid, Nautilus
Annelida
Marine segmented worms, earthworms, leeches
Nematoda
Heartworms, pinworms, many parasitic and free-living forms.
Hexapoda (Arthropoda)
Insects: beetles, ants, moths, butterflies, bees, dragonflies, cockroaches…
Arachnida (Arthropoda)
Spiders, scorpions, ticks, mites
B. Ancestry, Form and Function
1. Consider the synapomorphies common to both protostomes and deuterostomes.
Based on this information, what do you think the most recent common ancestor of these
organisms might have looked like? What characters did it have?
Probably an extremely simple, benthic vermiform (i.e., worm-shaped) beastie. Early signs of triploblasty,
with an incomplete digestive system (moth, no anus). Might have looked somewhat like an acoel or very
simple flatworm.
2. Based on your description of the protostome/deuterostome common ancestor, which organ
system(s) in these animals are likely the most primitive? (i.e., which evolved first?)
Integumentary (derived from ectoderm), digestive (derived from endoderm), nervous (derived from
ectoderm). Upon gastrulation, the seeds of these systems were sown.
3. Consider the synapomorphies common to all Ecdysozoans. Based on this information,
what do you think the most recent common ancestor of these organisms might have looked
like? What characters did it have?
Probably triploblastic with a pseudocoelom; vermiform with chitin tunic that was shed as the animal
grew. Spiral, determinate cleavage and other relevant protostome characters. (See lecture linked above)
4. Consider the synapomorphies common to all Lophotrochozoans. Based on this
information, what do you think the most recent common ancestor of these organisms might
have looked like? What characters did it have?
Probably triploblastic (or with very simple “promesodermal” mesenchyme), vermiform, trochophore-like
larval stage, typical protostome characters. (See lecture linked above)
5. List synapomorphies that set animals in each of the following taxa apart from the ancestral
Ecdysozoan (and from each other).
a. Nematoda – primarily molecular changes. Recent research suggests that
nematodes have secondarily lost many ecdysozoan traits in becoming
streamlined for a simple natural history.
b. Arthropoda – articulated exoskeleton, true coelom (lined on both sides with
mesoderm) reduced to form a gonocoel and pericardium; secondary development
of the hemocoel as the main body cavity (and main cavity of the open circulatory
system), complex hormones for ecdysis (shedding of the exoskeleton as the
animal grows), tagmosis (fusion of body segments), distinct cephalization
6. How does the main body cavity of a nematode differ from that of an arthropod?
List at least three key features.
• It is lined only on the parietal side by mesoderm
• Unlike the arthropod coelom, it is not derived via schizocoely
• Unlike the arthropod coelom, it is not segmental, but continuous throughout the
• body cavity.
• In the adult arthropod, the coelom is reduced to form only the pericardium and the gonocoel, and
does not form a hydrostatic skeleton as it does in nematodes.
7. List synapomorphies that set animals in each of the following taxa apart from the ancestral
Lophotrochozoan (and from each other).
a. Mollusca - headfoot; mantle, shell, visceral mass
b. Annelida - distinct metamerism retained throughout adulthood; molecular differences;
paired appendages on each segment
8. Both Mollusks and Arthropods have (1) an open circulatory system and (2) a reduced
coelom that functions as the pericardium (pear-ee-KAR’-dee-um) and gonocoel
(GONE’-oh-seal). If the hypothetical relationships in the tree above are correct, what does
this suggest about the evolution of these two characters in these distantly related phyla?
The two traits (open circulatory system and coelom forming a gonocoal/pericardium) may be convergent.
(But note that the same homeotic genes that direct the development of these structures may be operating
in these divergent taxa to produce the same types of structures.)
a. What defines a true coelom?
Lined on both the parietal and visceral surfaces with mesoderm.
b. What is a pericardium?
Coelomic lining surrounding the heart.
c. What is a gonocoel?
Coelomic lining surrounding the gonads and associated structures.
9. Consider the synapomorphies common to all Deuterostomes. Based on this
information, what do you think the most recent common ancestor of these organisms might
have looked like? What characters did it have?
A triploblastic, probably vermiform organism with a complete digestive tract; coelom formed via
enterocoely; radial, indeterminate cleavage, embryonic nervous system dorsal; embryonic circulatory
system ventral.
10. List synapomorphies that set animals in each of the following taxa apart from the ancestral
Deuterostome (and from each other).
a. Echinodermata
secondarily derived pentaradial symmetry; water vascular system; loss of excretory system
b. Hemichordata
pharyngeal gill slits, stomochord, proboscis
c. Chordata
notochord, post-anal tail, segmentally arranged muscle bundles, pharyngeal gill
slits
C. Practical Applications
1. A drug called lufenuron (loo-FEN’-yur-on) interferes with the activity of an enzyme known
as chitinase (KAI’-tin-ayze), which is involved in the normal formation of chitin (KAI-tin) in
growing arthropods. Lufenuron prevents normal maturation of animals that use chitin as
structural support, such as in the exoskeleton (arthropods) or a protective cuticle
(nematodes) surrounding the skin. Which of the following do you think would most likely be
adversely affected by medicating an infected host mammal with lufenuron?
a. fleas
d. heartworm (a nematode)
g. tapeworms
b. ear mites
e. ringworm fungus
h. ticks
c. leeches
f. liver flukes
i. caterpillars (not parasitic!)
2. It turns out that although lufenuron is effective against insects (Arthropoda, Hexapoda), it
does not kill ticks (Arthropoda, Arachnida).
Devise one or more logical hypotheses that might explain this.
• Ticks may have chitinase sufficiently different from that of other arthropods such that the drug
does not disable it.
• Ticks may have a mechanism to inactivate lufenuron, such as an enzyme that binds to it.
•
3. Animal phyla have long been classified into putatively monophyletic assemblages on the
basis of their body plans. However, as more sophisticated molecular techniques (nucleic acid
sequencing) have been applied to systematics, it has been discovered that shared
morphological characters do not always reflect recent common ancestry.
View the phylogenetic trees below.
Phylogeny based on morphology
Phylogeny based on molecular data
Now consider the following:
Ivermectin (EYE’-ver-mek-tin) is a macrolide (MAK’-ro-lide) drug produced from a
fungus (Streptomyces avermitilis) first isolated from a soil sample in Japan. Ivermectin is an
agonist for the neurotransmitter gamma-aminobutyric acid (GABA), a major
inhibitory neurotransmitter.
(NOTE:
an agonist increases the effects of the
neurotransmitter, whereas an antagonist reduces the effects.)
In mammals, GABA-containing neurons and receptors are found in the Central Nervous
System. In arthropods and nematodes GABA is found primarily in the Peripheral Nervous
System. This difference in location of GABA receptors is one reason why ivermectin can be
safely administered to (most) mammals for treatment of arthropod and nematode parasites.
Here’s how ivermectin works:
1. Ivermectin binds to a neuronal membrane, increasing release of GABA
2. GABA binds to the GABA-receptor-chloride channel complex of the postsynaptic
neuronal membrane. (Nerve impulses travel from the presynaptic neuron, across the
synapse to the postsynaptic neuron.)
3. The binding of ivermectin to the receptor complex causes an influx of chloride ions.
4. The abnormal influx of chloride ions hyperpolarizes the neuronal membrane.
5. The membrane becomes less excitatory
6. Nerve impulse transmission is decreased.
The hyperpolarization of neuronal membranes causes a fatal flaccid paralysis (FLA’-sid,
meaning floppy or loose, as opposed to paralysis caused by permanently contracted muscles)
in arthropods and nematodes.
Discuss the implications of this response to ivermectin in both arthropods and nematodes. Do
you think it is evidence of convergent evolution, or of homology? Explain your answer.
Probably homology, given the many synapomorphies (notably molecular) shared by nematodes and
arthropodds
Discussion
Can you think of other examples of characteristics used to devise phylogenies that might also
have relevance in treatment of disease, solution of environmental problems, or other practical
applications? Discuss.
Possible avenues to stroll:
Recall the dangers of assuming that evolutionary relationship automatically presumes similar physiology.
Example given: fipronil (active ingredient in Frontline) is safe for all mammals except lagomorphs
(rabbits and hares), in which it causes fatal neurological damage.
Consider relevance of pesticide
parasites/agricultural pests.
strategies
that
target
the
physiology
of
various
related
Insect growth regulators are a HUGE issue in pest management: finding a substance that inhibits the
growth/metamorphosis of insect pests without harming vertebrates or other beneficial animals (e.g.,
arachnids, such as spiders) means finding the unique “Achille’s Heel” of a particular taxon.
Synapomorphies of physiology can be a valuable tool in this ongoing battle.
(For example—Trypanosomes (not Animals, but you get the point) have a unique type of RNA known as
guide RNA (gRNA). Investigators hope to learn the mechanisms of action of this gRNA (which appears
to direct the rapid change of surface proteins in the parasite’s plasma membranes) and disable it,
crippling the parasite. Because the hosts don’t have gRNA (it’s not in their evolutionary package), such a
directed pesticide could be a safe treatment for African Sleeping Sickness. (Though we must always
beware of unexpected susceptibilities, such as described in the fipronil example above.)
Drive this discussion by trying to get students to see the economic value of understanding evolutionary
relationships as a potential KEY to unlocking the door to selective control of undesirable competitors,
parasites, and pests.