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Lecture Outline
Madeleine's Limbs
A. Modern humans are in a lineage of animal evolution that extends back millions of
years.
B. The fossils of the Burgess Shale in western Canada indicate a tremendous radiation
of animals just before the beginning of the Cambrian.
25.1
Overview of the Animal Kingdom
A. General Characteristics of Animals
1. Animals are multicellular (diploid) with tissues arranged into organs and organ
systems.
2. Animals are heterotrophic.
3. Animals require oxygen for aerobic respiration.
4. Animals reproduce sexually, and in some cases asexually, or both.
5. Most animals are motile during at least part of their life cycle.
6. Animal life cycles include a period of embryonic development; germ tissue
layers (ectoderm, endoderm, and in most species, mesoderm) give rise to adult
organs.
B. Variation in Body Plans
1. Animals with backbones are vertebrates (about 50,000 species); those without a
backbone are invertebrates (more than a million species).
2. Body Symmetry and Cephalization
a. Animals show either radial (round) or bilateral (right and left sides)
symmetry.
b. Cephalization means having a definite head end, usually with feeding and
sensory features.
3. Type of Gut
a. The place where food is digested is the gut; some are saclike with one
opening—a mouth.
b. “Complete” digestive tracts have two openings (mouth and anus) for
continuous food processing, often through specialized regions.
4. Body Cavities
a. A coelom (lined with peritoneum) is a space between the gut and body wall
that allows internal organs to expand and operate freely.
b. Some animals (flatworms) do not have a coelom but instead are packed
solidly with tissue between the gut and body wall.
c. Others, such as roundworms, have a “false” coelom (pseudocoel), not lined
with peritoneum.
5. Segmentation
a. A segmented animal is composed of repeating body units (for example, the
earthworm).
b. The segments may be grouped and modified for specialized tasks, as for
example in insects.
25.2
Puzzles About Origins
A. Animals probably arose from protistanlike ancestors about 900 million years ago.
B. Perhaps they arose from a colonial organism like Volvox .
1. The placozoan Trichoplax adhaerens is possibly typical of the earliest multicellular
animals.
2. It is a plate-shaped, two-layered animal with no symmetry and no organs.
25.3
Sponges–Success in Simplicity
A. Sponges (Phylum Porifera) are mostly marine animals of widely varying size and
shape.
B. Sponges have an asymmetric body with no true tissues, no organs.
1. Flattened cells cover the exterior.
2. Collar cells line the interior chambers.
a. By means of their beating flagella, these cells move large volumes of water in
through body pores and out through the large osculum at the top of the
body.
b. They also trap suspended food particles in their microvilli collars and
transfer the food to amoebalike cells in the matrix.
3. Between the two layers of cells there is a semifluid matrix with needlelike
spicules for support.
C. Sponges reproduce sexually and asexually.
1. Sperm are released into the surrounding water to be picked up by a nearby
sponge and directed to the egg within the matrix.
2. The zygote develops into a free-swimming larva.
3. Asexually, sponges reproduce by fragmentation and gemmules—a sporelike
collection of sponge cells that can survive until living conditions improve.
25.4
Cnidarians–Tissues Emerge
A. Cnidarians (Phylum Cnidaria) are radial, tentacled animals.
1. Most live in the seas; only a few species live in freshwater.
2. This phylum includes the hydrozoans Hydra and Obelia (hydrozoans), jellyfishes
(scyphozoans), and corals and sea anemones (anthozoans).
3. Unique stinging devices called nematocysts discharge threads to capture prey
and fend off predators.
B. Cnidarian Body Plans and Life Cycles
1. The medusa resembles an umbrella and floats like a tentacle-fringed bell in the
water; oral arms surround the central mouth.
2. The polyp is tubelike and is usually attached to some substrate; it may be solitary
or part of a colony.
3. The digestive cavity is saclike (only a mouth) and can accommodate prey larger
than the cnidarian itself.
4. An outer epidermis covers the body, and an inner gastrodermis lines the
digestive cavity.
a. A nerve net running through both layers coordinates the animal’s response
to stimuli.
b. Some jellyfishes also have sensory cells and contractile cells.
5. A jellylike mesoglea lies between the outer and inner body layers.
a. Jellyfishes have abundant mesoglea helpful in providing buoyancy and in
swimming.
b. Polyps have little mesoglea but use the water in their guts as a hydrostatic
skeleton.
6. The life cycle of a cnidarian may have a polyp and medusa stage, or it may have
just the polyp.
a. The medusa is usually the sexual form with either epidermal or
gastrodermal gonads.
b. The zygote develops into a swimming larva called a planula.
C. Examples of Cnidarian Diversity
1. Physalia is a free-floating colony of polyps and medusae specialized for feeding,
defense, and reproduction.
2. Corals (colonial organisms) secrete calcareous structures that accumulate to form
huge reefs.
25.5
Acoelomate Animals–and the Simplest Organ Systems
A. Animals from flatworms to humans have organs.
1. An organ is a grouping of tissues arranged in such a manner as to perform
specialized functions.
2. Interacting organs form an organ system.
B. Characteristics of Flatworms
1. The phylum Platyhelminthes includes the free-living turbellarians as well as the
parasitic flukes and tapeworms.
2. The gut is saclike with a single (mouth) opening through which a pharynx
extends for food gathering.
3. Most flatworms are hermaphrodites (both sexes in one body).
C. Major Groups of Flatworms
1. Turbellarians (planarians)
a. Protonephridia, with flame cells, regulate body fluid volume and
composition.
b. Asexual reproduction is by transverse fission.
2. Flukes
a. Trematodes are internal or external parasites of living vertebrate hosts.
b. Internal parasites require a primary host (such as a human) for sexual
reproduction and an intermediate host (such as a snail) for larval
development.
3. Tapeworms
a. Cestodes are intestinal parasites of vertebrates, where they absorb
predigested nutrients (they have no digestive tract).
b. The body consists of an anterior scolex solely for attachment to the host’s gut
and a string of proglottids, each of which possesses both male and female
organs.
25.6
Roundworms
A. Most members of the phylum Nematoda are small and free-living, but some are
parasitic on plants and animals (for example, hookworms in the human intestine).
B. They are bilateral and possess a slender tapered body.
1. The digestive tract is complete–mouth and anus for continuous food processing.
2. Reproductive organs lie in a false coelom filled with fluid.
3. A tough cuticle covers and protects the body.
25.7
Focus on Health: A Rogue's Gallery of Worms
25.8
Rotifers
A. Rotifers (phylum Rotifera) are microscopic, multicelled aquatic animals that
superficially resemble protozoans in size and movements.
B. Structurally they are remarkably complex, possessing a crown of cilia, jaws,
digestive tract, protonephridia, nerve cells, eyespots, and muscles.
25.9
A Major Divergence
A. Coelomate animals belong to two main groups:
1. Protostomes: molluscs, annelids, and arthropods.
2. Deuterostomes: echinoderms and chordates.
B. Protostomes are characterized by the following:
1. The early embryonic cell divisions are “angled”; this is called spiral cleavage.
2. In the ball-shaped early embryo, the first indentation (blastopore) becomes the
mouth.
3. The coelom forms from tissue at the sides of the gut near the blastopore.
C. Deuterostomes are characterized by the following:
1. The zygote divides symmetrically; this is called radial cleavage.
2. The blastopore becomes the anus; the mouth develops from a second opening.
3. The coelom forms from outpouchings of the gut wall.
25.10
A Sampling of Mollusks
A. Mollusks (phylum Mollusca) are the second largest invertebrate group.
1. Mollusca literally means "soft body."
2. Features in common usually include a head, foot, shell, mantle (skirtlike tissue
covering the body), ctenidia (gills), and a radula (a rasping tonguelike structure).
B. Major molluscan groups include:
1. Gastropods (literally “belly foot”) are the snails and slugs with coiled shells.
2. Chitons have eight plates covering the dorsal body.
3. Bivalves have a two-valved shell; clams are typical of this group.
4. Cephalopods–squids, octopuses, and nautili–are the swift-moving and smart members of
the phylum.
25.11
Evolutionary Experiments with Molluscan Body Plans
A. Twisting and Detwisting of Soft Bodies
1. As the gastropod larva develops, the internal organs undergo a unique twisting,
or torsion, that places the anus anteriorly near the mouth.
2. Nudibranchs seem to have undergone detorsion and have exposed ctenidia.
B. Hiding Out, One Way or Another
1. The chiton is protected when the eight plates roll around the soft, edible body.
2. A bivalve not only has its two protected shells but can also burrow in the sand to
disappear from sight of predators; water and suspended food are drawn in, and
waste voided, through openings called siphons, by the action of the cilia on the
gills.
C. On the Cephalopod Need for Speed
1. The body is modified for a highly active predatory life-style; it has tentacles and
beaklike jaws.
2. They move by a type of jet propulsion caused by mantle contractions.
3. To support greater activity, the circulatory system is a closed one; the nervous
system is well developed with a large brain; the eyes form images; and learning
and memory are possible.
25.12
Annelids–Segments Galore
A. This phylum includes earthworms, polychaetes, and leeches.
1. The body is bilateral with definite segmentation evidenced on the surface as
"rings."
2. Setae (bristles) which project directly from the highly segmented body may be
few or many in number.
B. Advantages of Segmentation
1. A segmented body has great evolutionary potential, for individual parts can
undergo modification and become highly adapted for specialized tasks.
2. Leeches have suckers at both ends; polychaetes have fleshy, paddle-like
parapods.
C. Annelid Adaptations–A Case Study
1. Earthworms bear few setae (oligochaetes).
2. Their habit of ingesting dirt particles while scavenging for organic matter makes
them valuable tillers of the soil.
3. The outer body surface is covered with a cuticle; internal partitions define
individual coelomic chambers filled with fluid to provide a hydrostatic skeleton.
4. The coelomic chambers in conjunction with the circular muscles, longitudinal
muscles, and setae allow the typical earthworm pattern of movement.
5. Paired nephridia occur in nearly every segment to regulate the volume and
composition of body fluids.
6. Paired nerve cords extend from the brain to run the length of the body with a
ganglion in each segment.
7. The circulatory system is closed (blood confined to hearts and vessels).
25.13
Arthropods—The Most Successful Organisms on Earth
A. Arthropod Diversity
1. This is the largest and most diverse phylum (Arthropoda).
2. It can be divided into four subphyla:
a. Trilobites: now extinct
b. Chelicerates: horseshoe crabs, spiders, scorpions, ticks, mites.
c. Crustaceans: copepods, crabs, lobsters, shrimps, barnacles.
d. Uniramians: centipedes, millipedes, insects.
B. Adaptations of Insects and Other Arthropods
1. Hardened exoskeleton:
a. The arthropod covering is a combination of protein and chitin (plus calcium
in some) that is flexible, lightweight, yet protective.
b. It is a barrier to water loss and can support a body deprived of water’s
buoyancy.
c. Exoskeletons restrict growth and so must be shed periodically (molting
process).
2. Jointed appendages:
a. Arthropod appendages are jointed.
b. Appendages became specialized for feeding, sensing, locomotion, sperm
transfer, and spinning silk.
3. Fused and modified segments:
a. Body segments became more specialized, reduced in number, and grouped
together.
b. In some lineages this has resulted in a head, thorax, and abdomen regions.
4. Respiratory structures:
a. Special tubes called tracheas supply oxygen directly to body tissues.
b. This allows high metabolic rates and sustained activity, as in flight.
5. Specialized sensory structures:
a. The compound eye provides a wide angle of vision.
b. Many individual units of the eye allow motion perception.
6. Division of labor:
a. Larval stages concentrate on feeding and growth.
b. Adults specialize in dispersal and reproduction.
c. The process of change from larva to adult is called metamorphosis.
25.14
A Look at Spiders and Their Kin
A. Chelicerates include some marine species such as horseshoe crabs and more familiar
arachnids.
B. Arachnids include spiders, scorpions, ticks, and mites.
1. Spiders are keen predators that trap insects in their webs produced as silk
strands from the abdomen.
2. Some mites are free-living, others are serious pests of plants and animals; ticks
are notorious blood-suckers and disease-carriers.
3. The chelicerate body bears eight legs plus a pair of chelicerae to pierce prey and a
pair of pedipalps to manipulate the food (or transfer sperm!).
4. Unique respiratory organs called book lungs reside in an invagination of the
abdominal wall; they greatly increase the surface area available for gas exchange.
25.15
A Look at the Crustaceans
A. Crustaceans get their name from the "crusty" exoskeleton.
B. Crustaceans include shrimps, crayfishes, crabs, barnacles, and pillbugs
1. These are important components of food webs and serve as human food also.
2. The crustacean body is divided into about twenty segments each of which bears
paired specialized appendages including antennae, mandibles, maxillae,
grasping claws, and legs.
3. The segmentation may be partially hidden in some segments because of a
smooth covering—the carapace.
25.16
How Many Legs?
A. Millipedes and centipedes have long, segmented bodies with many legs.
B. Millipedes are cylindrical, slow-moving vegetarians.
C. Centipedes have a more flattened body, move rapidly, and prey on small invertebrates using their fangs and venom glands.
25.17
A Look at Insect Diversity
A. Insects display many adaptations.
1. The body is divided into three regions: head (sensory and feeding), thorax
(locomotion by six legs, two pairs of wings), and abdomen.
2. The complete digestive tract is divided into three regions: foregut, midgut
(digestion and absorption), and hindgut (water reabsorption).
3. Unique Malpighian tubules process metabolic waste and aid in water retention.
B. Insect display enormous diversity.
1. Their great success is due to their ability to exploit nature’s resources during
different stages of metamorphosis, such as larvae, nymphs, and pupae.
2. Their ability to disperse by flight allows the utilization of widely ranging food
sources.
25.18
Focus on Health: Unwelcome Arthropods
25.19
The Puzzling Echinoderms
A. The name of this phylum, Echinodermata, refers to the “spiny skin” made of
calcium carbonate.
1. Members include sea stars, sea urchins, brittle stars, and sea cucumbers.
2. Adults are radially symmetrical; larvae are bilateral.
B. Internally, echinoderms are deuterostomes but of simple design.
1. The unique water-vascular system operates the tube feet by contracting the
ampulla on each one to achieve a suction useful in locomotion and prey capture.
2. Sea stars can evert their stomachs when feeding.