Chapter 30: Comparing
Invertebrates
Section 1: Evolution of the
Invertebrates
Evolution of the Invertebrates
• The evolutionary relationships between
different groups of organisms can be
shown in the form of a diagram called a
phylogenetic tree
– Shows our best understanding of
which phyla originate from a common
ancestor and approximately when
evolutionary lines diverged
Evolution of the Invertebrates
• The base of the tree represents the common
ancestor of all the groups shown on the tree
• Branches that originate close to the bottom of the
tree represent groups that evolved long ago
• Branches that originate near the top of the tree
represent groups that evolved relatively recently
• The tips of the branches represent living groups
• Some phylogenetic trees show “dead” branches
that do not reach the outside of the tree
• Dead branches represent extinct evolutionary lines
• There are no living groups from these lines
Evolution of the Invertebrates
• There are several major branches on a
phylogenetic tree
• Figure 30 – 3
– Protostomes, deuterostomes, acoelomates,
pseudocoelomates, and coelomates
• These branches represent basic
evolutionary lines in animals with
bilateral symmetry
Evolution of the Invertebrates
• The division of animals into
deuterostomes and protostomes is based
on events in early development
• The division of animals into
acoelomates, pseudocoelomates, and
coelomates is based on the structure of
the body cavity
Early Development
• Protostomes include flatworms, roundworms,
annelids, mollusks, arthropods, and the members
of most of the minor invertebrate phyla
• Deuterostomes include echinoderms, several
small phyla of strange-looking marine animals
we have not discussed, and all members of our
own phylum, Chordata
• To understand the reasons for dividing animals
into protostomes and deuterostomes, we must
examine the earliest stages in the development of
animals
Early Development
• Soon after an egg has been fertilized, it begins a series
of divisions
• These divisions lead first to a two-cell stage and then to
a four-cell stage
• When the embryo grows from four cells to eight cells,
the new cells can be arranged in different ways
• In spiral cleavage, which occurs in almost all
protostomes, the four new cells sit in between the four
older cells
• In radial cleavage, which occurs in almost all
deuterostomes, the four new cells sit directly on top of
the four older cells
Early Development
• In both protostomes and deuterostomes, the cells
of the embryo continue to divide until they form a
hollow ball
• Then the ball becomes flattened on one side and
folds in on itself
• The layer of cells on the outside of the ball is
called the ectoderm
• The layer of cells that has folded inside the ball is
called the endoderm
• Both the endoderm and ectoderm eventually
develop into several different kinds of tissue
Early Development
• The round central cavity enclosed by the endoderm
will become the digestive tract of the developing
embryo
• The opening of this cavity to the outside is called the
blastopore
• It is the blastopore that determines whether an animal
is a protostome or a deuterostome
– If the blastopore becomes the mouth, the animal is
a protostome
– If the blastopore becomes the anus and an opening
that appears later becomes the mouth, the animal is
a deuterostome
Early Development
• There is a third cell layer in embryos,
called the mesoderm, which is located
between the endoderm and the ectoderm
• Many important tissues, including
muscles, develop from the mesoderm
Body Cavities
• Body cavities are important for several reasons
– Provide a space in which internal organs can
be suspended so that they are not pressed on
by muscles and twisted out of shape by body
movements
– Allow room for internal organs to develop
and expand
– Contain fluids that may be involved with
internal transport, or the carrying of food,
wastes, and other materials from one part of
the body to another
Body Cavities
• Some phyla, such as flatworms, have no body cavity at
all
– Acoelomates
• Other phyla, such as roundworms, have a body cavity
that is partially lined with mesoderm
– Pseudocoelomates
• Still other phyla have a true coelom, or body cavity that
is completely lined with mesoderm
– Coelomates
• More advanced than the other two
• The complete mesoderm lining makes it possible
for the digestive tract to develop specialized
regions and organs, allows for the formation of
blood vessels, and makes it easier for complex
organ systems to develop
Chapter 30: Comparing
Invertebrates
Section 2: Form and Function in
Invertebrates
Form and Function in
Invertebrates
• Each animal phylum represents an experiment in
the design of body structures to perform the tasks
necessary for survival
• The appearance of each phylum in the fossil
record represents the random evolutionary
development of a basic body plan that is different
in some way from other body plans
• Evolution is random and undirected
Form and Function in
Invertebrates
• Organisms are not better or worse than one
another – they are simply different
• The body systems that perform the vital
functions of life have taken many different
forms in different phyla
• Some are complex, others are simple
• Some are efficient, others are not
• More complicated and efficient systems are
not necessarily “better” than simpler
systems
Movement
• Almost all animals use specialized tissues called
muscles to move
• Without muscles, animals could not swim, fly, burrow,
or run
• Muscles work only by contracting
• When muscles are stimulated, they generate force by
getting shorter
• When they are not stimulated, they relax
• In most animals, muscles work together with some sort
of skeletal system that provides firm support
• There are three main kinds of skeletal systems:
hydrostatic skeletons, exoskeletons, and endoskeletons
Hydrostatic Skeletons
• Hydrostatic skeletons do not contain hard
structures, such as bones or chitin plates, for
muscles to pull against
• Instead, the muscles surround and are
supported by a water-filled body cavity
• When the muscles contract, they push against
the water in the body cavity
• Cnidarians, some flatworms, roundworms,
some mollusks, and annelids have hydrostatic
skeletons
Exoskeletons
• Exoskeletons usually refer to the hard nonliving
coating that encloses an arthropod’s internal
organs and muscles
• However, the shells of mollusks can also be
considered exoskeletons
• Muscles attached to the inside of an arthropod’s
exoskeleton are used to bend and straighten the
joints
• Muscles attached to the shell in mollusks make it
possible for snails to withdraw into their shell and
for bivalves to close their two-part shell
Endoskeletons
• Endoskeletons are frameworks located
inside the body of animals
• Sponges, echinoderms, and vertebrates have
endoskeletons
• Animals with endoskeletons typically have
muscles that attach to the outside surface of
the endoskeleton
Feeding
• As you move through the invertebrate phyla
from simpler animals such as sponges to more
complex animals such as arthropods, you can
observe three major evolutionary trends
• First, simpler animals such as sponges,
cnidarians, and flatworms break down their
food primarily through intracellular digestion
• More complex animals use extracellular
digestion
Feeding
• In intracellular digestion food is digested, or
broken down, inside the cells
• In extracellular digestion, food is broken
down outside the cells – specifically, in a
digestive tract
• Mollusks, annelids, arthropods,
echinoderms, and chordates typically rely
on extracellular digestion
Feeding
• Second, cnidarians and some flatworms have a
simple digestive system that has a single
opening through which food enters and
through which solid wastes are expelled
• More advanced digestive systems, such as
those found in roundworms, mollusks,
annelids, arthropods, echinoderms, and
chordates, have two openings – a mouth at one
end and an anus at the other
Feeding
• Third, the digestive tract tends to acquire more
and more specialized regions
• The digestive system is not the only system to
become more specialized as you move from
simpler animals to more complex animals
• This evolutionary trend is seen in most of the
other systems responsible for performing
essential life functions
Internal Transport
• All cells of multicellular animals must be supplied
with oxygen and nutrients and must dispose of
metabolic wastes
• The smallest and thinnest multicellular animals
manage to fulfill their internal transport needs
through diffusion between their body surface and
the environment
• Most complex multicellular animals have a
collection of pumps and tubes called a circulatory
system
• There are two basic types of circulatory systems:
open and closed
Respiration
• In order to supply oxygen to and remove
carbon dioxide from their tissues, animals must
exchange these gases with the environment
• Two features are common to all respiratory
systems
• First, they almost always have structures that
maximize the amount of surface area in contact
with air or water
• Second, they have some way of keeping the
gas exchange surfaces moist so that diffusion
can occur
Respiration
• Some animals that live in water or in very moist
soil, such as cnidarians and flatworms, respire
through their skin
• Aquatic organisms – mollusks, crustaceans, some
insects, and many annelids, for example – have gills
that help them exchange gases with the water
around them
• Terrestrial invertebrates have evolved several organs
for breathing air
• These include the highly modified mantle cavities of
land snails, the book lungs of spiders, and the
tracheal tubes of insects
Excretion
• Multicellular animals, whether they live in
water or on land, must control the amount of
water in their tissues
• At the same time, all animals must get rid of
toxic nitrogenous wastes produced as a result
of cellular metabolism
• Excretory systems in invertebrates have
evolved in ways that enable these animals to
both regulate the amount of water in the body
and get rid of nitrogenous wastes
Excretion
• In all animals, the breakdown of amino
acids during cellular metabolism produces
ammonia
• Many aquatic animals simply allow
ammonia to diffuse through their body
tissues and out into the surrounding water,
which immediately dilutes it and carries it
away
Excretion
• Terrestrial animals must do two things: conserve
body water and get rid of nitrogenous wastes at
the same time
• In order to do this, many invertebrates convert
ammonia into urea
• Urea is soluble in water and is much less toxic
than ammonia
• The waste product produced by the excretory
system, which is called urine, is expelled from the
body
• Terrestrial animals can get rid of more wastes in
less water than their aquatic counterparts
Excretion
• Sponges, cnidarians, and roundworms
– Diffusion through body surfaces
• Freshwater flatworms
– Flame cells
• Insects and some arachnids
– Malpighian tubules
• Annelids, mollusks, and chordates
– Nephridia
Response
• Nervous systems gather information from
the environment, process information, and
allow animals to respond to it
• Invertebrates show three obvious trends in
the evolution of the nervous system:
centralization, cephalization, and
specialization
Response
• Cnidarians and some flatworms
– Nerve nets
• Other flatworms
– Ganglia
• Mollusks and arthropods
– Ganglia are organized into a brain
Reproduction and Development
• Many simple invertebrates reproduce asexually
through fragmentation or budding
• Asexual reproduction allows animals to produce
offspring rapidly from a single individual
• Sexual reproduction maintains genetic diversity
in a population
• Although sexual reproduction does not create
new genes, it does result in new combinations of
genes
• Most of the more complex animals reproduce
sexually
Fertilization
• There are two basic ways in which sperm cells
and egg cells are brought together in sexual
reproduction: external fertilization and
internal fertilization
• External fertilization is generally associated
with less complex animals
• The eggs are fertilized outside the body
• Internal fertilization is associated with more
complex animals
• The egg is fertilized inside the female’s body
Parental Care
• Many invertebrates do not take care of their
fertilized eggs or young
• The eggs are ignored as soon as they are
laid
• Most of the young are eaten or are exposed
to adverse environmental conditions and die
• Some invertebrates take care of their
offspring
Parental Care
• Some of the ways in which invertebrates care for their
offspring may seem horrifying to humans
• For example, the eggs of some species of mites hatch
within the female’s body
• The larvae immediately begin to devour their mother from
the inside!
• Within two days – while still inside their mother’s nearly
empty exoskeleton – the young mites mature, mate, and eat
their way to the outside
• The males die within a few hours
• The females seek out prey in the form of insect eggs and
begin to feed – even as their own offspring start chewing
on their internal organs