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Chapter 29
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Comparing Invertebrates
Summary
29–1 Invertebrate Evolution
Paleontologists have identified microscopic fossils from 570 to
610 million years ago. They identified trace fossils from the same
time period. Trace fossils are tracks and burrows made by softbodied animals.
Fossils of some primitive animals were discovered in the
Ediacara Hills of Australia. The Ediacaran animals lived 543 to
575 million years ago. They were flat and plate-shaped. They
lived on shallow sea bottoms and had soft, segmented bodies
and bilateral symmetry. The animals appear to have lacked cell
specialization or a front and a back end.
The Cambrian Period began 544 million years ago. It is
marked by many kinds of fossils. The Burgess Shale of Canada
is one of the best-known sites of Cambrian fossils. These animals
evolved complex body plans. Because of its great growth in
animal diversity, events of the early Cambrian Period are called
the Cambrian Explosion. The Burgess Shale animals typically had
body symmetry, segmentation, some type of skeleton, a front and
a back end, and appendages having many functions.
The appearance of each animal phylum in the fossil record
shows the evolution of a successful and unique body plan.
Modern sponges and cnidarians have little internal specialization.
As larger and more complex animals evolved, specialized cells
formed tissues, organs, and organ systems.
All invertebrates except sponges have some type of
body symmetry.
• Cnidarians and echinoderms have radial symmetry. Radial
symmetry is a body plan in which the body parts repeat
around the center of the body.
• Worms, mollusks, and arthropods have bilateral symmetry.
Bilateral symmetry is a body plan in which only a single,
imaginary line can divide the body into two equal halves.
A trend toward cephalization occurred with the evolution
of bilateral symmetry. Cephalization is the concentration
of sense organs and nerve cells in the front of the body.
Cephalization lets animals respond to the environment
in more sophisticated ways.
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Most complex animals are coelomates. They have a true
coelom. A coelom is a body cavity lined with tissue derived from
mesoderm.
• Flatworms are acoelomates—they have no coelom.
• Roundworms are pseudocoelomates. Their coelom is only
partially lined with mesoderm.
• Mollusks, annelids, arthropods, and echinoderms all have
a true coelom.
The zygote of most invertebrates divides to form a blastula.
An opening, called the blastopore, then forms in this blastula.
• In protostomes, the blastopore develops into a mouth.
Worms, arthropods, and mollusks are protostomes.
• In deuterostomes, the blastopore develops into an anus.
Echinoderms (and chordates) are deuterostomes.
29–2 Form and Function in Invertebrates
Biologists learn a great deal about the nature of life by comparing
the body systems of the various living invertebrates. All animals
perform the same essential tasks: feeding and digestion, respiration, circulation, excretion, response, movement and support, and
reproduction.
Feeding The simplest animals—sponges—break down food
inside their cells. Mollusks, annelids, arthropods, and echinoderms use extracellular digestion. Food is broken down outside
the cells of a digestive tract. Food enters the body through the
mouth, and wastes leave through the anus.
Respiration All respiratory systems share two features.
(1) Respiratory organs have a large surface area that is in
contact with air or water.
(2) Respiratory surfaces must be moist for diffusion to occur.
Aquatic mollusks, arthropods, and many annelids exchange gases
through gills. In land animals, surfaces are covered with water or
mucus. Such covering prevents water loss from the body. It also
moistens air moving through the respiratory system.
Circulation All cells need a constant supply of oxygen and
nutrients. They also must remove wastes. The smallest and
thinnest animals exchange materials with the environment by
diffusion. More complex animals use a system of pumps and
tubes for transport. There are two types of circulatory systems.
• In an open circulatory system, blood is only partly contained
within blood vessels. The blood moves through the vessels
into a system of sinuses, where the blood comes into direct
contact with the tissues.
• In a closed circulatory system, blood moves throughout the
body in vessels. The blood moves under force from a heart or
heartlike organ.
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Excretion Multicellular animals must control the amount of
water in their tissues. They also must get rid of ammonia, a toxic
nitrogen waste formed during metabolism. Most animals have
an excretory system to rid the body of metabolic wastes. The
excretory system also controls the amount of water in the tissues.
Aquatic organisms get rid of ammonia through diffusion. Many
land animals convert ammonia into urea. This compound is then
eliminated from the body in urine.
Response The more complex an animal’s nervous system is,
the more developed its sense organs are. Invertebrates show three
trends in the evolution of the nervous system: centralization,
cephalization, and specialization. Simple animals have nerve
cells that are spread through the body, while more complex
animals have centralized nerve cells. More complex animals
also have more highly specialized sense organs.
Support and Movement Invertebrates have one of three main
kinds of skeletal systems.
• Annelids and certain cnidarians have a hydrostatic skeleton.
In this system, muscles surround a fluid-filled body cavity
that supports the muscles. The muscles contract and push
against the water.
• Arthropods have an exoskeleton, or external skeleton.
• Echinoderms have an endoskeleton, a structural support
located inside the body.
Reproduction Some invertebrates may reproduce asexually.
However, most reproduce sexually at some part of their life
cycle. Asexual reproduction allows animals to reproduce rapidly.
Sexual reproduction maintains genetic diversity in a population
by creating individuals with new combinations of genes. Most
animals have separate sexes that produce eggs or sperm.
• In external fertilization, eggs are fertilized outside the
female’s body.
• In internal fertilization, eggs are fertilized inside the
female’s body.
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