Chap 32: The Diversity of Animal Forms
1
What is an animal: the basic characteristics
1. Animals are:
•
Heterotrophic (as opposed to plants that are autotrophic)
•
•
So they ingest their food
Multicellular
•
Protists such as amoeba and paramecium are unicellular
•
Plants can be unicellular or multicellular
•Eukaryotes
2
2. Animals lack cell walls
•
So how do they support themselves: structural proteins such
as collagen
3. Animals possess nervous and muscle tissue
4. Most animals reproduce sexually with a dominate diploid stage.
5. Early embryonic development (next slide)
3
Figure 32.1 Early embryonic development (Layer 1)
Cleavage: a series of rapid cell divisions and in most
cases leads to the formation of a blastula or hollow
ball of cells.
4
Figure 32.1 Early embryonic development (Layer 2)
5
Figure 32.1 Early embryonic development (Layer 3)
6
6. Hox genes: these are genes that control development. There is
diversity when these genes are expressed in a developing
embryo
and this produces the variety of animal forms.
7. Most biologist (systematists) are able to trace animal origins to a
single common ancestor (monophyletic)
•
A colony-based flagellated protist from Precambrian era
•
Probably related to the choanoflagellates
7
Figure 32.2 A modern choanoflagellate colony
Choanoflagellate colony inhabits ponds,
lakes and marine environments.
8
Figure 32.3 One hypothesis for the origin of animals from a flagellated protist
9
Figure 32.4 A traditional view of animal diversity based on body-plan grades
The circled numbers represent branch
points for each “major branch” or grade.
So for example, the presence of tissues is
a point of distinction between groups of
animals and is therefore a “grade.”
So what is really fun here is this is the
“traditional” view of animal phylogeny
or organization. There is a second type
of phylogeny based on rRNA.
10
Branch Point #1: Tissues or No Tissues
1. Tissue: a group of cells performing the same function.
•
Parazoans (“beside the animals”): lack true tissues
•
•
Example: sponges
Eumetazoans: possess true tissues of some sort.
•Examples: nervous tissue, muscle tissue, digestive
11
Branch Point #2: Type of Symmetry
So the eumetazoans, possessing tissues, can have their body plan
arranged one of two ways:
1. Radial Symmetry: hydras, jellies, sea anemones (Cnidarians)
•
Called radiata
•
Possesses a top (dorsal) and bottom (ventral) but no head or
rear end or left or right sides
•
Picture trying to slice a jelly fish into right and left sides
•
Diploblastic: possess only two germ layers (ectoderm and
endoderm)
•
Ectoderm: outer covering and CNS, hair, nails, sense
organs
•
12
Endoderm: lines digestive tube, liver, lungs, pancreas,
Figure 32.5 Body symmetry
13
Branch Point #2: Symmetry
2. Bilateral Symmetry: humans, crayfish, earthworms
•
Called bilateria
•
Possesses dorsal and ventral sides, head end (anterior) as
well as a rear end (posterior), and right and left side
•
Cephalization: concentration of sensory tissue near the
anterior end.
• Triploblastic: possesses a third germ layer, the mesoderm
that gives rise to muscles and bones, circulatory system,
kidneys
14
Branch Point #3: Body Cavity or No Body Cavity
The presence / absence of a body cavity is a division of the
bilaterally symmetrical animals.
1. Some of these animals do not have a cavity between the
digestive tract and the outer body wall
•
Acoelomates: solid body
•
Examples: flatworms such as planaria (Platyhelminthes)
2. Most of these triploblastic animals possess a body cavity
•
Coelomates: possess a fluid-filled space between
digestive tract and outer body wall.
•
The body wall is lined with mesoderm
•
Mesenteries suspend internal organs (like we have)
15
•
Earthworms (annelida), Chordates (humans)
Branch Point #3: Triploblastic organisms can be divided into coelomates, acoelomates and psuedocoelomates
2. (cont’d)
•
Pseudocoelomates: since the body cavity develops
differently, it is not completely lined by tissue from the
mesoderm.
•
Daphnia (rotifers) and roundworms
3. The value of a coelom
i.
Internal fluid cushions
ii. Internal fluid of earthworms is under pressure yet is
incompressible and serves as an internal skeleton
iii. Allows for independent movement of internal organs
16
Figure 32.6 Body plans of the bilateria
17
Branch Point #4: Protostome vs. Deuterostome
So those organisms with body cavities (coelomates) can have
differences. It is in their development where the differences lie.
There are 3 main differences:
1. Cleavage
•
Protostomes
•
Determinate cleavage: cell’s fate is determined early (4
cell stage)
•
Spiral cleavage: new cells lie in the grooves of the older,
lower cells.
18
Branch Pt #4: Protosome vs. Deuterostome
1. (cont’d)
•
Deuterostomes
•
Indeterminate cleavage: cells retain the capacity to
develop into a complete individual longer. This is how
humans can have identical twins and why embryonic
stem cells are so important.
•
Radial cleavage: cells align directly on top of each
other.
19
Branch Pt #4: Proto- vs. Deuterostomes
2. Coelom Formation: begins in the gastrula stage
•
Protostomes
•
•
These animals form splits in the mesoderm to form the
coelom
Deuterostomes
•
These animals form the coelom from folds in the
archenteron
20
Figure 32.10 Ecdysis
21
Branch Pt #4: Protostome vs Deuterostome
3. Fate of the Blastopore
•
The archenteron forms as the invagination continues during
gastrulation
•
The archenteron will extend through the entire blastula and
make another opening becoming the second opening of the
digestive tract.
•
Protostomes: blastopore forms the mouth, second
opening, the anus
•
Deuterostomes: mouth is the second opening, anus is the
blastopore.
22
Figure 32.7 A comparison of early development in protostomes and deuterostomes
23
Constantly Changing Animal Phylogeny
The small subunit ribosomal RNA or SSU-rRNA has been used to
show similarities and differences between views of animal
phylogeny
Where are there similarities between animal phylogeny based on
the traditional body plan arrangement and the more modern
molecular biology-based SSU-rRNA?
1. Branch #1: Presence (Eumetazoa) / Absence of Tissues
(Parazoa)
2. Branch #2: Radial vs. Bilateral Symmetry
3. Within the coelomates the deuterostomes are made of the
echinoderms (sea stars) and chordates (vertebrates like us)
24
What are the differences between between the traditional “body
plan” arrangement of animal classification and the SSU-rRNA
method?
The differences lie in the protostomes (animals without body
cavities). With the SSU-rRNA method:
1. The protostomes are divided into two groups or clades:
a. Lophotrochozoa: annelids and mollusks
 Possess a structure called a lophophore, ciliated
tentacles used for feeding (next slide)
b. Ecdysozoa: arthropods (secrete external skeleton)
 From “ecdysis”: shedding of old skeleton
Basic Concept: The annelids and mollusks go through a
similar larval stage called the trochophore larva as well
25
as having similar SSU-rRNA.
Figure 32.11 A lophophorate
26
Figure 32.9 A trochophore larva
27
Figure 32.8 Animal phylogeny based on sequencing of SSU-rRNA
28
2. Flatworms, which were classified as acoelomates and having
no body cavity, have a molecular-based similarity to
protostomes, which possess a body cavity.
a. It is thought that maybe they once had a coelom and then
lost it.
b. The pseudocoelomates, rotifers and nematodes, also are
placed within the protostomes
29
3. A New Phyla: Lophophorate
a. Examples: flatworms, roundworms, mollusks, segmented
worms.
b. Share characteristics with protostomes and deuterostomes
30
Figure 32.8 Animal phylogeny based on sequencing of SSU-rRNA
31
Figure 32.12 Comparing the molecular based and grade-based trees of animal
phylogeny- EGADS!!!!!
32
Origins of Animal Diversity
Occurred over a brief period of time
1. Lasted about 40 million years (565 to 525 million years ago)
2. During late Precambrian and early Cambrian periods
a. Fossils as old as 565 to 543 million yrs old have been
found in the Ediacara Hills of Australia (field trip?)
b. Fossilized animal embryos in China (570 million yrs ago)
c. Other fossils and molecular data even suggest 1 billion
years ago.
d. Types of animals fossilized: cnidarians (hydra-like) and
soft-bodied mollusks
33
3. Cambrian Explosion
a. 543 – 525 million years ago
b. First hard bodied animals
 Burgess Shale in British Columbia
 Greenland
 China
4. Because there were so many animals it is hard to sort out their
sequence of branching.
5. But why such an explosion of animal types, forms, etc.?
34
Figure 32.13x Burgess Shale fossils
35
Cambrian Diversification because of:
1. Change in biological communities: this lead to diverse
adaptations such as protective shells, new means of locomotion
all as an “attempt” to deal with a changing environment.
2. Oxygen levels may have increased to such a level that new and
more animals could survive and under aerobic metabolism. This
meant more active / aerobic means of locomotion so new
adaptations could be “tried.”
3. Evolution of Hox genes: these regulatory genes can lead to
diverse forms of body plans based on when they are active
during embryonic development of an animal form.
36