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biology
ch. 18 notes
“the evolution of invertebrate diversity”
Animal Evolution and Diversity
18.1
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Describe the defining characteristics of animals.
multicellular
heterotrophic
eukaryotes
*obtain nutrients by ingestion (*includes some
exceptions)
lack cell walls
collagen extracellular structural protein unique to
animals
*muscle for movement
nerves
18.2
Describe the five-stage hypothesis for the evolution
of animals from protists.
1. Colonial protist, an aggregate of identical cells
2. Hollow sphere of unspecialized cells
3. Beginning of cell specialization
4. Infolding
5. Gastrula-like “proto-animal”
18.2
Describe the Cambrian “explosion” of animal
diversity and two hypotheses that have been
advanced to explain its occurrence.
Dramatic increase in animal diversity with many animal
body plans and new phyla appearing in an evolutionarily
short time.
Hypothesis #1:
Ecological causes:
 evolution of hard body coverings
 led to increasingly complex predator-prey relationships
 and diverse adaptations for feeding , motility, and
protections.
Hypothesis #2:
Geologic changes:
Atmospheric oxygen high enough to support metabolism
of more active, mobile animals.
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Genetic framework was already in place:
the Hox complex of regulatory genes.
Much of variation comes from where and when Hox
genes are expressed within developing embryos.
Small genetic changes could have big effects.
(intensity of cause vs. magnitude of effect)
18.3
Explain how a hydrostatic skeleton helps an animal
keep its shape and move.
Hydrostatic skeleton: in soft-bodied animal, a
noncompressible fluid in the body cavity that provides a rigid
structure against which muscles contract, moving the animal.
Mrs. Loyd 
[email protected]
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18.3
List and describe four features that can describe an
animal’s body plan.
1. Symmetry:
 asymmetrical
o no symmetry, example: sponges
 radial:
o body parts radiate from the center.
o Animal has a top and bottom but not a right
and left.
o meets environment from all sides
o sessile, passively drifting.
 bilateral:
o mirror-image right and left sides
o distinct head (anterior)
o a tail (posterior)
o back (dorsal)
o bottom or belly side (ventral)
o cephalization facilitates mobility
o travel head-first so sense organs contact
environment first.
2. Tissues:
Tissues are collections of specialized cells, usually isolated
from other tissues by membrane layers, that perform specific
functions, example: nerve tissue.
Coelom?
No
PseudoTrue
Sponges
No true tissues
Cnidaria
Flat
worms
X
X
X
Round
worms
Annelids
Molluscs
Arthropods
X
X
X
X
No body cavity:
 no true tissues, example: sponges
 no fluid-filled space between the digestive tract and outer body wall,
example: Cnidaria
 body is solid with ectoderm and mesoderm, example: flatworms
Pseudocoelom:
body cavity not completely lined by mesoderm
functions like a coelom
example: roundworms (nematodes)
True Coelom:
Tissue layer derived from mesoderm
completely lines coelom
suspends internal organs (derived from endoderm)
Invertebrate Diversity
18.5-18.14 Describe the characteristics of and distinguish
between each of the phyla. Note several examples of
each phylum.
18.10 Define segmentation, explain its functions, and note the animal phyla
where it occurs.
Segmentation: the subdivision of the body along its length
into a series of repeated parts (segments)
Function:
Allows for greater flexibility
mobility
evolved as an adaptation facilitating movement
crawling and burrowing
Animal phyla:
Annelids (first segmentation genes)
Arthropods
Mrs. Loyd 
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18.12 Describe the process and significance of complete
metamorphosis.
Process:
 larval stage specialized for eating and growing.
 molts
 exists as an encased, non-feeding pupa
 body rebuilds from clusters of embryonic cells held in
reserve.
 Emerges as adult
 Specialized for reproduction and dispersal
Significance:
 specialization increases efficiency
 larva/adult eat different food
 avoids intergenerational competition
Complete vs. Incomplete Metamorphosis
Complete:
 larvae look very different than adult
 pupa and reorganization
 examples: butterflies and moths
Incomplete
 *larvae look like small adults
o examples: grasshoppers, roaches
 *larvae look, act different:
o dragonfly larvae (nyads) live in water
 multiple molts
 no pupa
18.16 Explain what we have learned about the evolution of
life from the study of “evo-devo.”
Novel features are easily evolved by small changes in master
control genes called homeotic genes.
“There has been an idea around biology for a very long time
that to get new things you would need new genes. New genes
for new structures, or new genes for new capabilities, new
genes for new types of animals…It doesn’t look like that at all.
It looks like a lot of things come about because very old genes
learn new tricks…the genes involved in building our eyes and
our hearts and our bones, these genes have been around for
500 million years and have been building all the rest of the
members of the animal kingdom.”
Sean Carroll
Mrs. Loyd 
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