Evolution of the Animal Phyla

Evolution of the
Animal Phyla
Fig. 21.2
Evolutionary
trends
among the
animals
21.6 Roundworms: The Evolution
of a Body Cavity
The evolution of an internal body cavity was
important for three reasons
Circulation
Rapid passage of material
Movement
Muscle-driven body movement
Organ function
Little deformation by surrounding muscles
Three kinds of body plans
Acoelomates
Have no body cavity
Pseudocoelomates
Have body cavity between
mesoderm and endoderm
Pseudocoel
Coelomates
Have body cavity entirely
within mesoderm
Coelom
Fig. 21.19
Phylum Nematoda
Nematodes are bilaterally symmetrical, cylindrical,
unsegmented worms
The pseudocoel serves as a hydrostatic skeleton
Gains rigidity from being filled with fluid under
pressure
Therefore muscles can work against this
“skeleton”
Lack a defined circulatory system
Covered by a thick, flexible cuticle
Rotifera
Food passes through the mouth by the sucking
action of the pharynx
Nematoda
Reproduction is sexual
Fig. 21.20
Fig. 21.21
21.7 Mollusks: Coelomates
The bulk of the animal kingdom consists of
coelomates
Development of specialized tissues in animals
involves primary induction
In this process one of the three primary tissues
interacts with another
A major advantage of the coelomate body plan is
that it allows mesoderm–endoderm contact
The three major groups
of mollusks
Fig. 21.22
Gastropods
Snails and slugs
Bivalves
Clams, oysters and
scallops
Cephalopods
Octopuses and squids
21.8 Annelids: The Rise
of Segmentation
Segmentation is the building of a body from a series
of similar segments
It offers evolutionary flexibility
Small change in existing segment can produce
a new segment with a different function
The first segmented animals to evolve were the
annelid worms, phylum Annelida
Most annelid species are marine
About one-third are terrestrial
Fig. 21.24
Earthworm
Shiny bristle
worm
The basic body plan is a tube within a tube
Three characteristics
Repeated segments
Separate segments able to expand or contract independently
Specialized segments
Front segments contain the worm’s sensory organs
Connections
Materials and information
pass through partitions in the
segments
Segmentation underlies the body
organization of all complex coelomate
animals
Fig. 21.25
Fig. 21.2
Evolutionary
trends
among the
animals
21.9 Arthropods: Advent of
Jointed Appendages
Arthropods belong to
the phylum Arthropoda
The most successful of
all animal groups
2/3rd of all named species
80% of all arthropods are
insects
Scientists estimate that a
quintillion insects are
Fig. 21.26
alive at any one time
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
Chelicerates
Arthropods that lack jaws, or mandibles
Their mouthparts, called chelicerae, are the foremost
appendages of the body
Mandibulates are arthropods that have mandibles
Jumping spider
Fig. 21.29
Bullfrog ant
Chelicerates
Arachnida is the largest class of chelicerates
57,000 named species
Largely terrestrial organisms
Spiders, ticks, mites, scorpions
Brown recluse
Black widow
Fig. 21.31
Poisonous spiders
Mandibulates
Crustaceans
Large, diverse group of primarily aquatic organisms
Differ from insects, but resemble millipedes and
centipedes, in having legs on their abdomen and thorax
Fig. 21.33
Body of a
lobster
Mandibulates
Crustaceans
Fig. 21.32
Edible crab
Three general types
1. Decapods
Crabs, shrimp, lobsters
2. Pillbugs and sowbugs
Terrestrial crustaceans
Sowbug
3. Barnacles
Sessile as adults
Mandibulates
Millipedes and Centipedes
Consist of a head region followed by numerous similar
segments
Centipedes have one pair of legs on each segment
Millipedes have two pairs of legs on each segment
Active
carnivores
Fig. 21.34
Sedentary
herbivores
Mandibulates
Insects
The largest group of arthropods by far
~ 1 million species identified
Fig. 21.36
Beetle
Flea
Honeybee
Grasshoppers
Dragonfly
Moth
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
Fig. 21.37