....

3. 21.1 General Features of Animals
• All animals are multicellular heterotrophs
– They all require oxygen for respiration
• Animals are diverse in form
– There are ~ 10 million living species
• ~ 99% are invertebrates (lacking a backbone)
• ~ 1% are vertebrates (possessing a backbone)
– There are about 36 phyla
• Most occur in the sea
• Three phyla dominate life on land
– Arthropoda; Mollusca; Chordata

4. • Animals lack cell walls
– They are usually quite flexible
• Animals are mobile
– They move more rapidly and in more complex ways than
members of other kingdoms
• Most animals reproduce sexually
• An animal develops from a zygote by a characteristic
process of embryonic development
– Morula  Blastula  Gastrula
– Details vary widely between phyla
• Provide clues to evolutionary relatedness

5. 21.2 The Animal Family Tree
• Taxonomists have traditionally created phylogenies
(family trees) based on two main criteria
– Anatomical features
– Embryonic development
The first branch: Tissues
Parazoa (“beside animals”)
Lack tissues and symmetry
Eumetazoa (“true animals”)
Possess tissues and symmetry
Fig. 21.3

6. • The second branch: Symmetry
– Eumetazoa have two branches
• Radiata
– Have radial symmetry
Bilateria
Have bilateral symmetry
Further branches
Based on key features of the body plan shared by all animals belonging to
that branch
This either-or nature has produced a family tree with a lot of paired branches
Fig. 21.3

25. • Sponges are the simplest animals
– Bodies consist of little more than masses of
specialized cells embedded in a gel-like matrix
• The adult sponge is shaped like a vase
– It is anchored in place on the seafloor
Fig. 21.7

26. • Sponges are perforated by tiny holes
– Basis of the Phylum name Porifera
• Unique flagellated cells called choanocytes or
collar cells, line the body cavity of the sponge
– Beating of the flagella draws water in through the
pores and drives it through the cavity
• The sponge is a ”filter-feeder”
• The choanocytes of sponges very closely resemble
a kind of protist called choanoflagellates
– These may be the ancestors of all animals

27. Fig. 21.8

28. Fig. 21.8

31. 21.4 Cnidarians: Tissues Lead to
Greater Specialization
• The structure of eumetazoans is much more
complex than that of sponges
• Radially symmetric eumetazoans form two distinct
embryonic layers
– An outer ectoderm  epidermis
– An inner endoderm  gastrodermis
– A jelly-like layer called the mesoglea forms
between the epidermis and gastrodermis

32. • There are two radially symmetric phyla
– Together, they are called Radiata
• 1. Cnidaria
2. Ctenophora
A minor phylum that includes the comb jellies
Fig. 21.9
Hydra
Jellyfish
Corals
Sea anemone

33. • Carnivores that capture their prey with tentacles
– Bear unique stinging cells called cnidocytes
• Contain a small but powerful harpoon called a
nematocyst
• A major evolutionary innovation among the radiates
is extracellular digestion of food
– In radiates, digestion begins in the gastrovascular
cavity
Cnidarians

34. Fig. 21.10

35. • Cnidarians have two basic body forms
Medusae
Free-floating, gelatinous and
often umbrella-shaped
Polyps
Cylindrical, pipe-shaped and
usually attached to a rock
Cnidarians may exist exclusively as either/or
Others alternate between the two phases
Fig. 21.11

36. Fig. 21.12 The life cycle of Obelia, a marine colonial hydroid

40. Fig. 21.14
• Bilaterally symmetrical animals have evolved a
definite head end, a process called cephalization
• Solid worms are the simplest of all bilaterally
symmetrical animals
Ectoderm
Mesoderm
Endoderm
Lack internal cavities,
except for digestive tract
Acoelomate
Nervous system
+

41. • Members of Platyhelminthes
– The largest phylum of solid worms
• Simplest animals in which
organs occur
• Some species are free-living
• Most species are parasitic
– Tapeworms
– Flukes
Flatworms
Many require two or more hosts
to complete their life cycle
Fig. 21.15a
Planaria

42. Fig. 21.16
Life cycle of
the human
liver fluke,
Clonorchis
sinensis

43. Fig. 21.17

44. • Have incomplete gut with only one opening
– Cannot eat, digest and excrete food simultaneously
• Have an excretory system consisting of a network
of tubules running throughout the body
• Have a simple nervous system
• Lack a circulatory system
• Most are hermaphroditic
Flatworms

52. 21.9 Arthropods: Advent of
Jointed Appendages
• Arthropods belong to
the phylum Arthropoda
• The most successful of
all animal groups
Fig. 21.26
80% of all arthropods are
insects
Scientists estimate that a
quintillion insects are
alive at any one time
2/3rd of all named species

53. • All arthropods have jointed appendages
• They have a rigid external skeleton made up of chitin
– This exoskeleton protects the animals and provides sites
for muscle attachment
• It is brittle, so its thickness limits arthropod body size
Arthropod bodies are segmented
Most larval stages
have many segments
These fuse into
functional groups
in the adult
Fig. 21.27

58. 21.10 Protostomes and
Deuterostomes
• In the coelomates there are two different
developmental patterns
– In protostomes, the mouth develops from or near
the blastopore
• The anus (if present) develops later from
another region of the embryo
– In deuterostomes, the anus develops from or near
the blastopore
• The mouth develops later from another region
of the embryo

59. Fig. 21.37

60. Fig. 21.37

61. • Deuterostomes differ from protostomes in three
other fundamental embryological features
1. Cleavage pattern
In protostomes, the egg cleaves spirally
In deuterostomes, the egg cleaves radially
2. Developmental fate of cells
In protostomes, the cells are committed even early on
In deuterostomes, the commitment occurs later
3. Origination of coelom
In protostomes, it forms directly from the mesoderm
In deuterostomes, it forms indirectly via the archenteron

62. 21.11 Echinoderms:
The First Deuterostomes
• Echinoderms have an endoskeleton composed of
hard calcium-rich ossicles that are often fused
• They consist of about 6,000 living marine species
Sea star
Fig. 21.38 Sea cucumber
Feather star
Sand dollar
Sea urchin

63. • Echinoderms are bilaterally symmetrical as larvae
– But they become radially symmetrical as adults
• This could be an environmental adaptation
• Adults have a five-part body plan
• The key evolutionary innovation is the development
of a water vascular system
– A fluid-filled system with a central ring canal and
five radial canals
• Thousands of tiny, hollow tube feet extend from
each radial canal

64. Fig. 21.39

76. • 1. Amniotic egg
– Watertight egg
– Contains four membranes
2. Dry skin
Covers body and prevents water loss
3. Thoracic breathing
Increases lung capacity
Chorion: Allows O2 entry
Amnion: Fluid-filled cavity
Yolk sac: Provides food
Allantois: Excretes waste
Plus, leg arrangement to better support body weight
Fig. 22.17

82. Fig. 22.22
Other Characteristics of Modern Mammals
Placenta
Brings the bloodstream of mother and
fetus into close contact
The two don’t mix
Characteristic of most
mammals