33.1. How Animals Evolved and Are Classified (p. 576)
A. Animals Are Multicellular Eukaryotes
1. Animals are heterotrophic by ingestion. (Fig. 33.1)
2. Unlike fungi that rely on external digestion, animals usually digest their food in a central cavity.
3. Animals produce heterogametes (eggs and sperm) and follow the diplontic life cycle in which the adult is diploid.
4. Meiosis produces haploid gametes that join to form a zygote that develops into an adult.
5. Animals probably evolved from a protistan ancestor, most likely a protozoan. (Fig. 33.2) [transp. 173]
a. There are approximately 34 animal phyla; we consider nine major invertebrate phyla and the chordates.
b. All major animal phyla are found in Cambrian fossils, but it is difficult to trace a fossil history because earlier animals did not fossilize well.
c. The evolutionary relationships have been worked out largely based on a study of animal anatomy.
B. Classification Criteria (Fig. 33.2) [transp. 173]
1. Body shape is described as types of symmetry.
a. Asymmetry means a body plan has no particular symmetry.
b. Radial symmetry describes body parts that are arranged around a central axis, like spokes of a wheel.
1) Radially symmetrical animals tend to be sessile (i.e., attached to a substrate or less motile).
2) This type of symmetry enables an animal to reach out in all directions from one center.
c. Bilateral symmetry describes a body having a right and left, or complementary halves.
1) One longitudinal cut down the center produces mirror halves.
2) Bilaterally symmetrical animals tend to be active and to move forward at an anterior end.
2. Germ layers refer to the number of layers of tissues.
a. Most animals are made of three tissue layers: endoderm, ectoderm and mesoderm.
b. Animals with only ectoderm and endoderm are limited to a tissue level of organization.
c. Animals with all three tissue layers can develop an organ level of organization.
3. Body cavities are different.
a. The acoelomates lack a body cavity or coelom; a coelom is a body cavity lined by mesoderm.
b. Pseudocoelomates possess a pseudocoelom; the body cavity is incompletely lined by mesoderm.
c. Coelomates are animals that possess a coelom completely lined with mesoderm.
4. Coelomates are either protostomes or deuterostomes.
a. Protostomes are coelomates whose embryonic development shows a blastopore associated with a mouth.
b. Deuterostomes are coelomates whose embryonic development shows a blastopore associated with anus, and a second opening becomes the mouth.
5. Segmentation occurs in certain coelomate animals (e.g., annelids, arthropods, and chordates).
33.2. Multicellularity Evolves (p. 578)
A. About 5,000 species of sponges are in the phylum Porifera.
1. Sponges have no symmetry and no tissues, and are believed to be out of the mainstream of animal evolution.
2. Sponges remain at the cellular level of evolution, may have evolved separately from protozoan ancestors.
3. Sponges are aquatic, largely marine animals, that vary greatly in size, shape, and color.
4. Their saclike bodies are perforated by many pores; Porifera means "pore-bearing."
5. Cellular organization is demonstrated by fragmenting a sponge; cells survive and reorganize into a sponge.
6. Several types of cells are found in a sponge. (Fig. 33.3) [transp. 174]
a. The outer layer of the wall of a sponge contains flattened epidermal cells; some have contractile fibers.
b. The middle layer is a semifluid matrix with wandering amoeboid cells and spicules.
c. The inner layer is composed of collar cells (choanocytes).
d. There are no nerve cells or means of coordination between cells.
7. Beating collar cells produce water currents that flow through pores in sponge wall into a central cavity and out through an osculum, the upper opening; simple sponges 10 cm tall filter as much as 100 liters of water a day.
8. Sponges are sessile filter feeders, organisms that stay in one place and filter food from the surrounding water.
a. Collar cells engulf and digest microscopic food particles in food vacuoles, or pass them to amoeboid cells.
b. Amoeboid cells transport nutrients from cell to cell, and they also produce gametes and spicules.
9. Sponges can reproduce asexually by budding, which can produce quite large colonies, or by fragmentation which occurs when sponges are chopped up; each piece can start a complete sponge.
10. Sponges can reproduce sexually: eggs and sperm are released into a central cavity; the zygote develops into a ciliated larva.
11. Sponge classification is based on the chemistry of its skeleton composed of spicules.
a. Spicules are tiny needle-shaped structures with one to six rays, depending on the chemical structure.
b. Chalk sponges have spicules composed of calcium carbonate.
c. Glass sponges have spicules of silica.
12. Spongin is a protein made of modified collagen, found in bodies of sponges that gives a sponge its flexibility.
33.3. Tissue Layers Evolve (p. 580)
A. Comb jellies are in the phylum Ctenophora. (Fig. 33.4b)
1. Comb jellies are small (a few cm), develop only ectoderm and endoderm, and are also radially symmetrical.
2. Comb jellies are transparent and often luminescent; their eight plates of fused cilia resemble long combs.
3. Most of their body is a jellylike packing material called mesoglea.
4. They are the largest animals to be propelled by beating cilia.
5. They capture prey by long tentacles covered with sticky filaments, or by entire body covered by sticky mucus.
B. Cnidaria Have Radial Symmetry
1. About 9,000 species of cnidarians are in the phylum Cnidaria. (Figs. 33.4a, 33.5) [transp. 175]
2. Cnidaria also develop only ectoderm and endoderm.
3. These tubular or bell-shaped animals live in coastal waters; there are also oceanic jellyfish, freshwater hydras.
4. Only cnidaria have cnidocytes, a specialized cell that contains a nematocyst. (Fig. 33.6) [transp. 176]
a. The nematocyst is a fluid-filled capsule, which contains a long, spirally coiled hollow thread.
b. When the trigger of the cnidocyte is touched, the nematocyst is discharged.
c. Some threads merely trap a prey or predator; others have spines that penetrate and inject paralyzing toxins.
5. The body of a cnidarian is a two-layered sac with the epidermis derived from ectoderm.
6. The inner tissue layer (derived from endoderm) secretes digestive juices into the gastrovascular cavity, which digests food and circulates nutrients. (Fig. 33.5, 33.6) [transp. 175 and 176]
7. Muscle fibers are at the base of the epidermal and gastrodermal cells.
8. Nerve cells located below epidermis near mesoglea interconnect and form a nerve net throughout the body.
a. In contrast to highly organized nervous systems, the nerve net allows transmission of impulses in several directions at once, which could result in multiple firings of nematocysts in parts not directly stimulated.
b. Having both muscle fibers and nerve fibers, these animals are capable of directional movement, the body can contract or extend, and the tentacles that ring the mouth can extend to grasp prey.
9. Two basic body plans are seen among cnidaria. (Fig. 33.5) [transp. 175]
a. A polyp is a vase-shaped, sedentary body plan, in which the mouth is directed upward.
(Fig. 33.5a, b, c)
b. A medusa is a bell-shaped, motile body plan, in which the mouth is directed downward.
(Fig. 33.5a, d, e)
c. A medusa has more mesoglea than a polyp, and the tentacles are concentrated on the margin of the bell.
d. Both body forms may have been a part of life cycle of all cnidaria; some exhibit alternation of generations.
e. When both stages are present, the polyp stage is sessile and produces medusae.
f. The medusan stages is motile and produces egg and sperm, dispersing the species.
C. Cnidarian Diversity
1. Sea anemones and corals are in the class Anthozoa. (Fig. 33.5b, c)
a. Sea anemones are solitary polyps 5-100 mm in height and 5-200 mm in diameter or larger.
b. They are often brightly colored and look like flowers.
c. The thick, heavy body rests on a pedal disk and supports a mouth surrounded by hollow tentacles.
d. Sea anemones feed on various invertebrates and fish.
e. They attach to rocks, timbers, etc., or may be mutualistic with hermit crabs, living attached to crab's shell.
f. Corals may be solitary but most are colonial.
g. Most are found in shallow waters; the accumulation of their calcium-carbonate remains builds reefs.
2. Class Hydrozoa includes hydras and Portuguese man-of-war (Physalia). (Fig. 33.5d)
a. The polyproid stage is dominant.
b. Portuguese man-of-war is a colony of polyps; original polyp is a gas-filled float.
c. Other polyps specialize for feeding or reproduction.
d. It can cause serious injury to swimmers with a tentacle having numerous nematocysts; each tentacle arises from the base of each feeding polyp.
3. The class Schyphozoa includes the true jellyfishes (e.g., Aurelia).
a. The medusal stage is dominant in jellyfish; the polyp remains small.
b. Jellyfishes also serve as food for larger marine animals.
D. Hydra and Obelia in Depth
1. Hydra are solitary, freshwater hydrozoan polyps.
a. The body is a small tube about one-quarter inch in length.
b. Four to six tentacles surround the mouth, the only opening at one end.
c. Hydra can move from one location by gliding or even somersaulting.
d. Hydras have both muscular and nerve fibers, and respond to touch.
2. Hydra anatomy
a. Epidermal cells are termed epitheliomuscular cells and contain muscle fibers.
b. Cnidocytes and sensory cells are also present in the epidermis.
c. Interstitial cells can produce an ovary or testis, and may assist regeneration.
d. Gland cells secrete digestive juices into the gastrovascular cavity; tentacles capture and stuff prey into it.
e. Digestion is completed within food vacuoles of nutritive-muscular cells.
f. Nutrients diffuse to the rest of the body.
3. Hydras reproduce both asexually (budding) and sexually.
a. Hydras reproduce asexually by budding.
b. In sexual reproduction, sperm from a testis swim to an egg within an ovary; after early development within the ovary, a protective shell allows it to survive until conditions are optimum for it to emerge.
4. Obelia is a colony of polyps enclosed in a hard, chitinous covering.
a. Feeding polyps have nematocysts and can extend beyond the covering to capture tiny worms, etc.
b. The polyps are connected and partially digested food is distributed through the colony.
c. The colony can increase in size by asexually budding new polyps.
d. Reproductive polyps bud off the medusae stage, which can be free-living or attached to the colony.
e. Obelia medusae have tentacles equipped with nematocysts; gastrovascular cavity extends into tentacles.
f. The nerve net runs in two rings around the bell margin; it is supplied with statocysts and ocelli.
g. Sperm and egg from the medusae stage unite; resulting zygote develops into a ciliated planula larva. (Fig. 33.7b) [transp. 177] [micro. slide 58]
5. Evolutionary History of Cnidaria
a. Some biologists propose that a planuloid-type organism gave rise to both the cnidaria and the flatworms.
b. Cnidaria are diploblastic (two germ layers), radially symmetrical animals exhibiting a tissue level of organization, whereas flatworms are triploblastic (three germ layers), bilaterally symmetrical animals exhibiting an organ level of organization.
33.4. Bilateral Symmetry Evolves (p. 584)
A. Triploblasts
1. All animals beyond this point are triploblasts with three germ layers and adults at the organ level of organization.
2. Flatworms have a sac body plan while ribbonworms have a tube-within-a-tube body plan.
3. The tube-within-a-tube plan allows specialization of parts along the tube.
B. About 650 species of marine ribbon worms are in the phylum Nemertea.
1. Ribbon worms have a distinctive eversible proboscis stored in a rhynchocoel; when the walls of the rhynchocoel are contracted, the proboscis is everted.
2. The eversible proboscis is a long, hollow tube that can be everted and shot outward through a pore located just above the mouth; used primarily for prey capture, and for defense, locomotion, and burrowing. (Fig. 33.8)
C. Flatworms Are Flat
1. About 13,000 species of flatworms belong to the phylum Platyhelminthes.
2. Classification: planaria and relatives are freshwater animals in the class Turbellaria; flukes are external or internal parasites in the class Trematoda; and tapeworms are internal parasites in the class Cestoda.
3. In addition to endoderm and ectoderm, a mesoderm layer gives rise to muscles and reproductive organs.
4. There is no coelom; they are acoelomates.
5. A branched gastrovascular cavity is site of extracellular digestion and distributes nutrients about the body.
6. Gas exchange occurs by diffusion.
7. An excretory system functions as an osmotic-regulating system.
8. Flatworms are bilaterally symmetrical, and free-living forms exhibit cephalization.
9. Flatworms have ladder-type nervous system; paired ganglia form a brain; sensory cells are in the body wall.
D. Planaria Are Free Living
1. Turbellaria include freshwater planaria such as Dugesia. (Fig. 33.9) [transp. 178][micro. slide 59]
2. Planaria live in lakes, ponds, and streams and feed on small living or dead organisms.
3. The head is arrow-shaped; the side extensions are sensory organs to detect food and enemies.
4. Two light-sensitive eyespots have pigmentation that makes them look cross-eyed.
5. Three muscle layers---outer circular layer, inner longitudinal layer, and diagonal layer---allow varied movement.
6. In larger forms, locomotion is accomplished by movement of cilia on the ventral and lateral surfaces.
7. Numerous gland cells secrete a mucous material upon which the animal moves.
8. Digestion
a. The animal captures food by wrapping itself around prey, entangling it in slime, and pinning it down.
b. The pharynx is a muscular tube that extends through the mouth and through which food is ingested.
c. In a three-branched gastrovascular cavity, digestion is extracellular and intracellular. (Fig. 33.9b)
9. Excretion
a. The flame-cell system consists of a series of interconnecting canals that run length of the body on either side of the longitudinal axis and side branches of the canals, each ending in a flame cell. (Fig. 33.9c)
b. The flame cell is a bulb-shaped cell containing a tuft of cilia inside the hollow interior of the bulb; the cilia move back and forth, bringing water into the canals that empty through pores at the body surface.
c. It functions in both water excretion and osmotic regulation; typical of freshwater, free-living flatworms.
10. Planaria can reproduce both sexually and asexually.
a. They can constrict beneath the pharynx and each half will grow into a whole animal-regeneration.
b. Planaria are hermaphroditic, possessing both male and female sex organs.
c. Planaria cross-fertilize each other.
d. Fertilized eggs are enclosed in a cocoon and hatch in two to three weeks.
E. Parasitic Flatworms Cause Serious Illnesses
1. Parasitic animals, such as flukes and tapeworms, have characteristic modifications.
a. With loss of predation comes loss of cephalization; a head carries hooks and suckers to attach to the host.
b. There is extensive development of the reproductive system with loss of other systems.
c. A well-developed nervous and gastrovascular system are not needed; it does not need to seek out or digest prey.
d. Flukes and tapeworms are covered by a tegument that protects them from host digestive juices.
2. Two Hosts
a. Both flukes and tapeworms use secondary or intermediate hosts to transport the species from primary host to primary host.
b. The primary host is infected with the sexually mature adult; the secondary host contains the larval stage(s).
F. Flukes Invade Organs
1. The class Trematoda includes flukes.
2. Blood, liver, and lung flukes inhabit those organs.
3. Fluke bodies tend to be oval and elongate.
4. They lack a definite head but have an oral sucker surrounded by sensory papillae; another sucker helps attach.
5. Flukes have reduced digestive, nervous, and excretory systems.
6. Reproductive systems are well developed and usually hermaphroditic.
7. The blood fluke causes schistosomiasis.
a. Schistosomiasis is a disease found predominantly in tropical Africa and South America.
b. Unlike most flukes, blood flukes are male or female; flukes deposit eggs in blood vessels around intestine.
c. The eggs migrate to the intestine and are passed out with feces.
d. The tiny larvae hatch in water and swim about until they detect and enter a particular species of snail.
e. The larvae reproduce asexually and eventually leave the snail.
f. If larvae penetrate the skin of the human body, they begin to mature in the liver, and implant in the small intestine blood vessels.
g. A weakened person is then more likely to die from secondary diseases.
8. The Chinese liver fluke requires two intermediate hosts.
a. Humans become infected when they eat uncooked fish.
b. Adults migrate to the liver and deposit eggs in the bile duct, which carries the eggs to the intestine.
c. The larval flukes must then pass through two intermediate hosts, a snail and a fish.
G. Tapeworms Live in the Gut (Fig. 33.11)[transp. 179]
1. The tapeworms scolex contains hooks and suckers for attachment to intestinal wall of host.
(Fig. 33.11) [transp. 179]
2. Behind the head is a short neck and then a long string of proglottids.
3. Each proglottid segment contains a full set of both male and female sex organs and little else.
4. There are excretory canals but no digestive system and only rudiments of nerves.
5. After fertilization, proglottids become a bag of eggs; mature proglottids break off and pass out with feces.
6. If the eggs of tapeworms are ingested by pigs or cattle, the larvae become encysted in the muscle of the hosts.
7. The covering of ingested eggs is digested away and the larvae burrow through the intestinal wall and travel by bloodstream to lodge and encyst in muscle; a cyst is a hard-walled structure sheltering a larval worm.
8. If humans eat the meat of infected pigs or cattle and fail to cook it properly, they too become infected.
33.5. A Pseudocoelom Evolves (p. 588)
A. Rotifers and Roundworms Are Pseudocoelomates
1. A pseudocoelom is a body cavity that is incompletely lined with mesoderm; structural support is provided by the hydrostatic pressure of the fluid in the pseudocoelom against a tough cuticle.
(Fig. 33A) [transp. 180]
2. Rotifers belong to the phylum Rotifera, with about 2,000 species known.
a. The digestive tract is the inner tube and the rest of the animal is the outer tube.
b. Rotifers are microscopic and abundant in freshwater.
c. A crown of cilia (corona) forms a rotating wheel; it serves as organ of locomotion and brings food to mouth.
3. Ascaris Infects Humans (Fig. 33.12)[transp. 181]
a. These worms are unsegmented, with a smooth outside wall.
b. The worms move by whiplike motions because only longitudinal muscles lie within the pseudocoelom.
c. Mating produces eggs that mature in the soil; therefore, most are limited to warmer climates.
d. When eggs are swallowed, larvae burrow through the intestinal wall, moving to the liver, heart and/or lungs.
e. Within the lungs, larvae molt and, after 10 days, migrate up the windpipe to throat where they are swallowed.
f. In the intestine, the mature worms mate and the female deposits eggs that are lost with the feces.
g. Feces must reach the mouth of the next host to complete the life cycle; proper sanitation prevents infection.
B. Other Roundworm Parasites
1. Trichinosis is a serious infection.
a. Humans contract Trichinella eating raw pork containing encysted larvae. (Fig. 33.12) [micro. slide 60]
b. After maturation, the female adult burrows into the wall of the small intestine and produces living offspring that are carried by the bloodstream to the skeletal muscles where they encyst.
2. Filarial worms cause various diseases.
a. Dirofilaria, the heartworm of dogs, is a common filarial worm of the temperate zones.
b. Elephantiasis occurs in tropical Africa; caused by a filarial worm that utilizes mosquito as a secondary host.
1) The adult worms reside in and block lymphatic vessels; ultimately results in limbs of an infected individual swelling to monstrous size. (Fig. 33.13)
2) Elephantiasis is treatable in its early stages but not after scar tissue has blocked lymphatic vessels.