Inveterate Zoology

1. BIOL 201:
Invertebrate Zoology
Chapter 4:
Introduction to
Metazoa
Rob Swatski
Asst. Prof. Biology
HACC-York1

2. Metazoans
Animals in Kingdom
Animalia
Multicellular
1-30 million species in
29 major phyla
Colonial protozoan
ancestors are
choanoflagellates (sister
taxon) 2

3. Metazoan
Characteristics
Eukaryotic
heterotrophs
Larger bodies:
costs/benefits?
Most motile (even
sessile adults have
motile larvae)
Anterior-posterior
polarity; oral-aboral
3

4. Anterior
Posterior4

5. Oral
5
Aboral

6. Metazoan
Ground
Plan
Specialized cells
organized as tissues
(division of labor)
Primary (original)
tissues: epithelial &
connective
Epithelium: sheets of
cells bound to each
other by cell-adhesion
molecules
Cuticle: surface coat of
glycoproteins or
secreted extracellular
matrix (ECM) 6

7. 7

8. Connective
Tissue
Widely separated,
nonadjoining cells in
ECM of proteoglycan
gel with collagen
proteins
Skeleton: support,
protection, muscle
attachment
Exoskeleton:
thickened outer
cuticle with proteins
or minerals
Endoskeleton: internal
stiffening of ECM with
cross-linking
(cartilage) or mineral
secretion (bone)
8

9. Asexual
(Clonal)
Reproduction
Budding
Fragmentation
Fission
Parthenogenesis:
development of
individual from an
unfertilized egg (virgin
birth)9

10. Fragmentation 10

11. Parthenogenesis 11

12. Sexual
Reproduction
Diploid adults form
haploid gametes
(egg & sperm) via
meiosis
Fertilization 
diploid zygote
Zygote divides by
mitosis into an
embryo (ontogeny)
Embryo divides by
cleavage  early
cells called
blastomeres
12

13. Embryo
Development
Blastula: 1-cell layer
thick hollow or solid ball
of cells
Gastrulation:
invagination of one wall
of blastula  2-layered
gastrula
Morphogenesis
Ectoderm & endoderm
(primary germ layers)
form around gelatinous
blastocoel
13

14. 14

15. Indirect
Development
Life cycle includes a
larval stage
Larva: independent
stage with different
anatomy & niche
Biphasic life cycle:
benthic adult &
planktonic larva 
value?
Larva settles in suitable
habitat & undergoes
metamorphosis into an
immature juvenile 15

16. Direct
Development
Embryo develops directly
into juveniles without a
larval stage
Juvenile resembles a
miniature version of the
adult (grasshoppers)
Considered to be a
derived trait
Indirect development
with external fertilization
& planktonic larva is the
ancestral pattern
16

17. Reproductive
Adaptations
Improve chances of
fertilization & embryo
survival: increase
synchrony & proximity
Hermaphroditism:
common in species
with small population
densities & sessile
lifestyles
Any nearby individual is
a potential mate
Most hermaphrodites
cross-fertilize rather
than self-fertilize 
why? 17

18. 18

19. Maternal
Provisioning
Adaptations providing
physical protection &
nutrients to offspring are
very valuable
Oviparous: eggs spawned
before or just after
fertilization
Viviparous: internal
fertilization, embryos
gestate in maternal body,
& release larva or
juveniles
Brooding: eggs released
from mother, but are
retained on or taken back
into her body
19
Oviparous
aphids

20. Viviparous pea aphids
20

21. Spider with her
brood
21

22. Body Size &
Division of
Labor
Most metazoans are 0.5
mm – 1 m in size
Prokaryote (seed) 
Protozoan (grapefruit) 
Animal (stadium)
Cell specialization
improves efficiency
Requires functional
compartmentalization &
cellular integration
(started with protozoans)
22

23. 23
Hierarchy of
Functional
Compartments
Organism
Organ system
Organ
Tissue
Cell
Organelle

24. Surface Area-
to-Volume
Ratio
SA:V is significantly
affected by increases
in body size
As a cell grows larger,
its area is squared &
its volume is cubed
Eventually, SA (supply)
will not be able to
support cell volume
(demand)
Limits exchange of
gases, nutrients, &
wastes 24

25. 6:1 0.6:1 0.006:1
25

26. Adaptations
That Increase
SA:V
Microvilli,
pseudopodia, &
internal membranes
Tissues arranged as
thin 2-D sheets around
a metabolically inert
ECM core
Body plans that are flat
& thin or long &
slender
Fractal body plans:
tubes in tubes
26

27. 27

28. 28

29. Size &
Transport
Rates of diffusion slow
drastically over great
distances
Effective diffusion
distance is roughly 0.5
mm for most animals
Body diameters larger
than 1 mm may be
diffusion-limited
Circulatory system
needed for bodies larger
than 1 mm in diameter
29

30. 30

31. 31

32. Size &
Metabolism
Metabolic rate
increases with body size
However, 1g of shrew
tissue consumes more
power than 1g of
elephant tissue
Poikilotherms (cold-
blooded animals)
consume 8 times more
mass-specific energy
than protozoans
Homeotherms
(mammals & birds)
consume 29 times more
energy than a
poikilotherm of equal
mass 32

33. Advantages of
Large Body
Size
Mass-specific decrease in
metabolic rate
Reduced risk of predation
by protozoa
Larger metazoans can
prey upon protozoans
Motile metazoans move
faster than protozoans
Multicellularity allows
ability to regenerate
33

34. Ontogeny &
Phylogeny
Metazoan ontology
includes developmental
stages subject to natural
selection
Heterochrony: changes
in the timing of
developmental events –
allows potential for
evolutionary change
Two types of
heterochrony
Pedomorphosis &
Peramorphosis
34

35. Pedomorphosis
A trait of a descendent
species resembles an
ancestral larval or
juvenile developmental
trait
Results in smaller &
simpler descendents
with short generation
times
Common in species
living in unpredictable
or changing
environments
Adapted to colonize
entirely new habitats
(interstitial descendents
of polychaete worms)
35

36. 36

37. 37

38. Cockroach nymphs
38

39. Peramorphosis
A trait of a descendent
species that develops
beyond the ancestral trait
Results in larger & more
complex descendents
with longer generation
times
Favored in constant or
predictable environments
(deep sea, coral reefs)
Larger body size is a major
trend in metazoan
evolution
39

40. Colossal Squid (Mesonychoteuthis)
12-14 m long!
40

41. Origins of
Metazoa
Colonial theory:
Metazoans are derived
from colonial flagellated
protozoans
Most widely accepted
theory, supported by
morphological &
molecular data
Choanoflagellates &
Metazoa are sister taxa
Spherical colony of
flagellated cells divided
by mitosis, but daughter
cells held within ECM
41

42. Proterospongia sp.
42

43. 43

44. 44
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