C H A P T E R 1 9

GENERAL FEATURES OF ANIMALS
• Animals share many important characteristics, such as they:
• are heterotrophs
• are multicellular and lack cell walls
• can move from place to place
• have diverse forms and habitats
• reproduce, mostly by sexual reproduction
• have a common pattern of development
• have unique tissues

THE ANIMAL FAMILY TREE
• Animals consist of 35 very different phyla.
• To judge which phyla are more closely related, taxonomists traditionally have compared anatomical features and aspects of embryological development.
• The end result are phylogenies , which are basically like family trees.

THE ANIMAL FAMILY TREE
• The kingdom Animalia is traditionally divided into two main branches based on tissue presence.
• Parazoa possess neither tissues nor organs and have no discernible symmetry.
• They are represented mostly by the phylum
Porifera, the sponges.
• Eumetazoa have a definite shape and symmetry and, in most cases, tissues organized into organs and organ systems.
• Although very different, the Parazoa and
Eumetazoa are thought to have evolved from a common ancestor.

THE ANIMAL FAMILY TREE
• Eumetazoans are divided into two groups:
• Radiata have radial symmetry and two embryological layers, an outer ectoderm and an inner endoderm .
• This body plan is called diploblastic.
• Bilateria have bilateral symmetry and a third embryological layer, the mesoderm , that occurs between the ectoderm and the endoderm.
• This body plan is called triploblastic .

THE ANIMAL FAMILY TREE
• Traditionally, the Bilateria have been subdivided based on traits that were important to the evolutionary history of phyla.
• For example, the presence or absence of a body cavity.
• The traditional animal phylogeny relies on the either-or nature of categories.

No tissues
Parazoa
Radial symmetry
Radiata
No coelom
Acoelomates
Pseudocoel
Pseudocoelomates
Protostomes
Coelom forms from mesoderm cell mass
Mix of characteristics
Coelomates
Mix of characteristics
Coelom forms from embryonic gut
Deuterostomes
Sponges Cnidarians Ctenophorans Flatworms Nematodes Mollusks Annelids Arthropods Lophophorates Echinoderms Chordates

THE ANIMAL FAMILY TREE
• The traditional animal phylogeny is being revised because some of the important characters may not be conserved to the extent previously thought.
• Molecular systematics offers a means to construct phylogenic trees using unique gene sequences as means to detect relatedness.
• New molecular data has resulted in a variety of new phylogenies, most of which divide protostomes into:
• Lophotrochozoans
• Ecdysozoans

No tissues
Parazoa
Radial symmetry
Radiata
Grow by increasing body mass, locomotion is ciliary, trochophore larvae
Lophotrochozoa
Protostomes
Grow by molting
Ecdysozoa
Coelom forms from embryonic gut
Deuterostomes
Sponges Cnidarians Ctenophorans Lophophorates Flatworms Mollusks Annelids Nematodes Arthropods Echinoderms Chordates

SIX KEY TRANSITIONS IN BODY
PLAN
• The evolution of animals is marked by six key transitions in body plan:
1.
tissues
2.
bilateral symmetry
3.
body cavity
4.
segmentation
5.
molting
6.
deuterostome development

SIX KEY TRANSITIONS IN BODY
PLAN
• The presence of tissues is the first key transition in the animal body plan.
• Only the Parazoa, the sponges, lack defined tissues and organs.
• These animals exist as aggregates of cells with minimal intercellular coordination.
• All other animals, the Eumetazoa, possess tissues.

SIX KEY TRANSITIONS IN BODY
PLAN
• Virtually all animals other than sponges have a definite shape and symmetry.
• Radial symmetry is a body plan in which all parts of the body are arranged around a central axis.
• Any plane passing through the central axis divides the organism in halves that are approximate mirror images.
• Bilateral symmetry is a body plan with distinct right and left halves that are mirror images.
• The plan allows for specialization among body regions and more efficient movement.

SIX KEY TRANSITIONS IN BODY
PLAN
• The evolution of a body cavity was an important step in animal evolution.
• This internal space allowed for the support of organs, distribution of materials, and coordination of development.
• For example, the digestive tract can be larger and longer.

SIX KEY TRANSITIONS IN BODY
PLAN
• Bilateral animals can be divided into two groups based on differences in the basic pattern of development.
• Protostomes include the flatworms, nematodes, mollusks, annelids, and arthropods.
• Deuterostomes include the echinoderms and the chordates.
• Deuterostomes evolved from protostomes more than 630 million years ago.

SIX KEY TRANSITIONS IN BODY
PLAN
• The subdivision of the body into segments is another key transition in the animal body plan.
• In highly segmented animals, each segment can develop a more or less complete set of adult organ systems.
• Each segment can function as a separate locomotory unit.

Parazoa
Radiata
Acoelomates Pseudocoelomates
Protostomes
Eumetazoa
Bilateria
Segmented
Coelomates
Deuterostomes
Segmented
EVOLUTIONARY
TRENDS AMONG
THE ANIMALS
Radial symmetry
No true tissues
Molting
Pseudocoel
Multicellularity
Jointed appendages exoskeleton, molting
Segmentation
Notochord, segmentation, jointed appendages
?
Deuterostome development, endoskeleton
Coelom
Body cavity
Bilateral symmetry
Tissues
Ancestral protist

TABLE 19.2
THE MAJOR ANIMAL PHYLA
Phylum
Arthropoda
(arthropods)
Typical Examples
Insects, crabs, spiders, millipedes
Key Characteristics
Most successful of all animal phyla; chitinous exoskeleton covering segmented bodies with paired, jointed appendages; most insect groups have wings; nearly all are freshwater or terrestrial
Approximate
Number of
Named Species
1,000,000
Mollusca
(mollusks)
Snails, clams, octopuses, nudibranchs
Chordata
(chordates)
Mammals, fish, reptiles, birds, amphibians
Platyhelminthes
(flatworms)
Planaria, tapeworms, flukes
Nematoda
(roundworms)
Ascaris, pinworms, hookworms, Filaria
Annelida
(segmented worms)
Earthworms, marine worms, leeches
Soft-bodied coelomates whose bodies are divided into three parts: head-foot, visceral mass, and mantle; many have shells; almost all possess a unique rasping tongue called a radula; most are marine or freshwater but 35,000 species are terrestrial
110,000
Segmented coelomates with a notochord; possess a dorsal nerve cord, pharyngeal pouches, and a tail at some stage of life; in vertebrates, the notochord is replaced during development by the spinal column; most are marine, many are freshwater, and 20,000 species are terrestrial
56,000
Segmented coelomates with a notochord; possess a dorsal nerve cord, pharyngeal pouches, and a tail at some stage of life; in vertebrates, the notochord is replaced during development by the spinal column; most are marine, many are freshwater, and 20,000 species are terrestrial
20,000
Pseudocoelomate, unsegmented, bilaterally symmetrical worms; tubular digestive tract passing from mouth to anus; tiny; without cilia; live in great numbers in soil and aquatic sediments; some are important animal parasites
20,000
Coelomate, serially segmented, bilaterally symmetrical worms; complete digestive tract; most have bristles called setae on each segment that anchor them during crawling; marine, freshwater, and terrestrial
12,000

TABLE 19.2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(CONTINUED)
Approximate
Numberof
NamedSpecies
Phylum
Cnidaria
(cnidarians)
Typical Examples
Jellyfish, hydra, corals, sea anemones
Key Characteristics
Soft, gelatinous, radially symmetrical bodies whose digestive cavity has a single opening; possess tentacles armed with stinging cells called cnidocytes that shoot sharp harpoons called nematocysts; almost entirely marine
10,000
Echinodermata
(echinoderms)
Sea stars, sea urchins, sand dollars, sea cucumbers
Porifera
(sponges)
Barrel sponges, boring sponges, basket sponges, vase sponges
Soft, gelatinous, radially symmetrical bodies whose digestive cavity has a single opening; possess tentacles armed with stinging cells called cnidocytes that shoot sharp harpoons called nematocysts; almost entirely marine
6,000
Asymmetrical bodies without distinct tissues or organs; saclike body consists of two layers breached by many pores; internal cavity lined with food-filtering cells called choanocytes; most marine (150 species live in freshwater)
5,150
Lophophorates
(Bryozoa, also called moss animalsor
Ectoprocta)
Bower bankia,
Plumatella, sea mats, sea moss
Rotifera
(wheel animals)
Rotifers
Microscopic, aquatic deuterostomes that form branching colonies, possess circular or U-shaped row of ciliated tentacles for feeding called a lophophore that usually protrudes through pores in a hard exoskeleton; Bryozoa are also called Ectoprocta because the anus, or proct, is external to the lophophore; marine or freshwater
4,000
Small, aquatic pseudocoelomates with a crown of cilia around the mouth resembling a wheel; almost all live in freshwater
2,000

SPONGES: ANIMALS WITHOUT
TISSUES
• Sponges are members of the Phylum
Porifera .
• Their bodies a little more than masses of specialized cells embedded in a gel-like matrix.
• Clumps of cells disassociated from a sponge can give rise to new sponges.

SPONGES: ANIMALS WITHOUT
TISSUES
• The body of a sponge is perforated by many pores.
• Choanocytes are flagellated cells that line the body cavity of the sponge and draw in water through the pores.
• The sponge is a filter feeder which traps any food particles.

CNIDARIANS: TISSUES LEAD TO
GREATER SPECIALIZATION
• Radiata are radially symmetrical and include two phyla.
• Cnidaria comprises the hydra, jellyfish, corals, and anemones.
• Ctenophora comprises the comb jellies.
• The members of the Radiata have a body plan that allows them to interact with their environment on all sides.

CNIDARIANS: TISSUES LEAD TO
GREATER SPECIALIZATION
• A major evolutionary advance in the
Radiata is extracellular digestion of food.
• Digestion begins outside of cells in a gut cavity called the gastrovascular cavity .
• This form of digestion allows animals to digest an animal larger than itself.

CNIDARIANS: TISSUES LEAD TO
GREATER SPECIALIZATION
• Cnidarians (phylum Cnidaria) are carnivores that capture prey with tentacles that ring their mouths.
• These tentacles and, sometimes, the body surface bear stinging cells called cnidocytes .
• Within each cnidocyte is a harpoonlike barb, called a nematocyst , which cnidarians use to spear prey and retract it towards the tentacle
• The nematocyst can discharge so explosively that it is capable of piercing the hard shell of a crab

CNIDARIANS: TISSUES LEAD TO
GREATER SPECIALIZATION
• Cnidarians have two basic body forms:
• Medusae are a floating form.
• Polyps are a sessile form.
Epidermis
Mesoglea
Gastrodermis
Tentacles
Gastrovascular cavity
Medusa
Mouth
Gastrovascular cavity
Polyp

CNIDARIANS: TISSUES LEAD TO
GREATER SPECIALIZATION
• Medusae are often called “ jellyfish, ” because of their gelatinous interior, or
“ stinging nettles, ” because of their nematocysts.
• Polyps are pipe-shaped animals that usually attach to rock.
• In corals, the polyps secrete a deposit of calcium carbonate in which they live.

Feeding polyp
Medusae
Testis
Ovary
Sexual reproduction
Medusa bud
Reproductive polyp
Eggs Sperm
Zygote
Blastula
Mature colony
Young colony and asexual budding
Settles down to start new colony
Free-swimming planula larva

SOLID WORMS: BILATERAL
SYMMETRY
• Body symmetry differs among the
Eumetazoa.
• Radial symmetry means that multiple planes cutting the organism in half will produce mirror images.
• Bilateral symmetry means that only one plane can cut the organism in half to produce mirror images.

SOLID WORMS: BILATERAL
SYMMETRY
• Most bilaterally symmetrical animals have evolved a definitive head end.
• This process is termed cephalization .

SOLID WORMS: BILATERAL
SYMMETRY
• The bilaterally symmetrical eumetazoans produce three embryonic layers.
• Ectoderm will develop into the outer coverings of the body and the nervous system.
• Mesoderm will develop into the skeleton and muscles.
• Endoderm will develop into the digestive organs and intestine.

SOLID WORMS: BILATERAL
SYMMETRY
• The solid worms are the simplest of all bilaterally symmetrical animals.
• The largest phylum of these worms is the
Phylum Platyhelminthes , which includes the flatworms .
• Flatworms lack any internal cavity other than the digestive tract.
• This solid condition is called acoelomate .
• Flatworms are the simplest animals in which organs occur.

SOLID WORMS: BILATERAL
SYMMETRY
• Most flatworms are parasitic but some are free-living.
• Flatworms range in size from less than a millimeter to many meters long.

SOLID WORMS: BILATERAL
SYMMETRY
• There are two classes of parasitic flatworms:
• Flukes
• Tapeworms
• The parasitic lifestyle has resulted in the eventual loss of features not used or needed by the parasite.
• For example, parasitic flatworms do not need eyespots.
• This loss of features that lack adaptive purpose for parasitism is sometimes called degenerative evolution.

Flukes often require two or more hosts to complete their life cycles
Metacercarial cysts in fish muscle
Cercaria
Raw, infected fish is consumed by humans or other mammals
3
2
Bile duct
Liver
1
Redia
Adult fluke
Egg containing miracidium
Miracidium hatches after being eaten by snail
Sporocyst

SOLID WORMS: BILATERAL
SYMMETRY
• Tapeworms are a classic example of degenerative evolution.
• The body of a tapeworm has been reduced to two primary functions.
• Eating
• Reproduction

SOLID WORMS: BILATERAL
SYMMETRY
• Those flatworms that have a digestive cavity, have an incomplete gut, one with only one opening.
• The gut branches throughout the body and is involved in both digestion and excretion.
• The parasitic flatworms lack a gut entirely and absorb food directly through their body walls.

SOLID WORMS: BILATERAL
SYMMETRY
• Flatworms lack a circulatory system and all cells must be within diffusion distance of oxygen and food.
• Flatworms have a simple nervous system, either a nerve net or longitudinal nerve cords with cross connections.
• Free-living forms have eyespots to distinguish light from dark.

ROUNDWORMS: THE EVOLUTION OF
A BODY CAVITY
• A key transition in the evolution of the animal body plan was the evolution of the body cavity.
• The evolution of an internal body cavity helped improve the animal body design in three areas:
• circulation
• movement
• organ function

ROUNDWORMS: THE EVOLUTION OF
A BODY CAVITY
• There are three basic kinds of body plans found in bilaterally symmetrical animals:
• Acoelomates have no body cavity.
• Pseudocoelomates have a body cavity (called a pseudocoel ) located between the mesoderm and the endoderm.
• Coelomates have a body cavity (called a coelom ) that develops entirely within the mesoderm.
Acoelomate
Pseudocoelomate
Coelomate
Ectoderm
Mesoderm
Endoderm
Ectoderm
Mesoderm
Endoderm
Pseudocoel
Ectoderm
Mesoderm
Coelomic cavity
Endoderm

ROUNDWORMS: THE EVOLUTION OF
A BODY CAVITY
• The largest pseudocoelomate phylum is
Nematoda, containing about 20,000 species.
• The members of this phylum include nematodes, eelworms, and other roundworms.
• Nematodes are unsegmented, cylindrical worms covered by a flexible cuticle that is molted as they grow.
• Nematodes move in a whiplike fashion.
http://youtu.be/tp-O3LME3OU

ROUNDWORMS: THE EVOLUTION OF
A BODY CAVITY
• Some nematodes are parasitic in humans, cats, dogs, and animals of economic importance.
• Heartworm in dogs is caused by a nematode
• Trichinosis is an infection caused by the nematode Trichinella and is transmitted to humans who eat undercooked pork.
• Intestinal roundworms, Ascaris lumbricoides, live in human intestines.

ROUNDWORMS: THE EVOLUTION OF
A BODY CAVITY
• Another pseudocoelomate phylum is Rotifera.
• Rotifers are small, aquatic organisms that have a crown of cilia at their heads.
• The cilia help in both locomotion and feeding.
http://youtu.be/cYNJOVDQexA

MOLLUSKS: COELOMATES
• The body of a mollusk (Phylum Mollusca) is comprised of three regions:
• a head-foot
• a visceral mass containing the body ’ s organs.
• a mantle that envelopes the visceral mass and is associated with the gills.

MOLLUSKS: COELOMATES
• There are three major groups of mollusks:
• Gastropods —include the snails and slugs.
• Bivalves —include clams, oysters, and scallops.
• Cephalopods —include the octopuses and squids.

MOLLUSKS: COELOMATES
• Mollusks have a unique feeding structure, called a radula .
• The radula is a rasping tonguelike organ that bears rows of pointed, backward-curving teeth.

MOLLUSKS: COELOMATES
• In most mollusks, the outer surface of the mantle secretes a protective shell.
• The shell has multiple layers comprised of protein, calcium, and pearl.

ANNELIDS: THE RISE OF
SEGMENTATION
• One of the early innovations in body plan to arise among the coelomates was segmentation .
• Segmentation is the building of a body from a series of similar segments.
• This body plan offers a lot of flexibility in that small changes to segments can produce a new kind of segment with different functions.
• The first segmented animals to evolve were the annelid worms, Phylum Annelida .

ANNELIDS: THE RISE OF
SEGMENTATION
• The basic body plan of an annelid is a tube within a tube.
• The digestive tract is suspended within the tube of the coelom.
• There are three body plan characteristics:
• Repeated segments
• Specialized segments
• Connections

ARTHROPODS: ADVENT OF JOINTED
APPENDAGES
• The most successful of all animal groups is the Phylum Arthropoda , consisting of the arthropods .
• These animals have jointed appendages.
• In addition to joints, arthropods have an exoskeleton made of chitin.
• The muscles of arthropods attach to the interior of this outer shell.
• The shell offers protection against predators and water loss.

ARTHROPODS: ADVENT OF JOINTED
APPENDAGES
• Chitin cannot support much weight.
• Arthropod size is limited as a result.
• Arthropod bodies are segmented like annelids.
• Segments often fuse into functional groups in the adult stage.
Antennae
Mouth

ARTHROPODS: ADVENT OF JOINTED
APPENDAGES
• Chelicerates are arthropods that lack jaws.
• They include spiders, mites, scorpions, and horseshoe crabs.
• Mandibulates are arthropods with jaws, called mandibles .
• They include the crustaceans, insects, centipedes, and millipedes.
Antenna
Eye
Eyes
Pedipalp
Chelicera Mandible

ARTHROPODS: ADVENT OF JOINTED
APPENDAGES
• The chelicerate fossil record goes back 630 million years.
• A surviving type of chelicerate from this period is the horseshoe crab.

ARTHROPODS: ADVENT OF JOINTED
APPENDAGES
• The class Arachnida has 57,000 named species of arachnids , including spiders, ticks, mites, scorpions, and daddy longlegs.
• Arachnids have a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs.

ARTHROPODS: ADVENT OF JOINTED
APPENDAGES
• Crustaceans belong to the subphylum Crustacea and comprise a diverse group of mandibulates.
• There a 35,000 species of crustaceans described including species of crabs, shrimps, lobsters, crayfish, water fleas, pillbugs, and sowbugs.

ARTHROPODS: ADVENT OF JOINTED
APPENDAGES
• Most crustaceans have two pairs of antennae, three pairs of chewing appendages, and various numbers of legs.

ARTHROPODS: ADVENT OF JOINTED
APPENDAGES
• Crustaceans pass through a larval stage called the nauplius .

ARTHROPODS: ADVENT OF JOINTED
APPENDAGES
• Millipedes and centipedes have bodies that consist of a head region followed by numerous similar segments.
• Centipedes have one pair of legs per segment while millipedes have two.
• Centipedes are all carnivorous while millipedes are herbivorous.

ARTHROPODS: ADVENT OF JOINTED
APPENDAGES
• Insects belong to the
Class Insecta and are the largest group of arthropods.
• They are the most abundant eukaryotes on the earth.
• Insects have three body sections:
• Head
• Thorax
• Abdomen

PROTOSTOMES AND
DEUTEROSTOMES
• There are two major kinds of coelomate animals representing two distinct evolutionary lines:
• Protostomes
• The mouth develops from or near the blastopore.
• Deuterostomes
• The anus forms from or near the blastopore; the mouth forms on another part of the blastula.
Mesoderm Anus Coelom
Protostomes
Archenteron Coelom
1 cell 2 cells
Deuterostomes
4 cells 8 cells 16 cells 32 cells
1 cell 2 cells 4 cells 8 cells 16 cells 32 cells
Blastula
Blastula
Mouth
Blastopore Mesoderm splits
Archenteron
Mouth forms from
Coelom blastopore
Mouth
Coelom
Blastopore Archenteron outpockets to form coelom
Anus
Anus forms from blastopore

PROTOSTOMES AND
DEUTEROSTOMES
• Deuterostomes also differ from protostomes in three other fundamental ways:
• The pattern of cleavage
• Protostomes have spiral cleavage while deuterostomes have radial cleavage.
• Fating of cells
• Occurs later in deuterostome cleavage than in protostome cleavage.
• Origin of the coelom.

ECHINODERMS: THE FIRST
DEUTEROSTOMES
• Echinoderms are deuterostomes that belong to the phylum
Echinodermata.
• Echinoderm means “ spiny skin ” and refers to the endoskeleton of calcium-rich ossicles just beneath the echinoderm ’ s skin.
• Entirely marine animals and include sea stars, sea urchins, sand dollars, and sea cucumbers.
• All are bilaterally symmetrical as larvae but become radially symmetrical as adults.

ECHINODERMS: THE FIRST
DEUTEROSTOMES
• A key adaptation of echinoderms is the water vascular system that aids movement.
• This system is fluid-filled and composed of a central ring canal from which five radial canals extend out into the arms.
http://youtu.be/8JOxiT5_zpc
• From each radial canal, tiny vessels extend through short side branches into thousands of tiny, hollow tube feet .
• Echinoderms can extend the tube feet, attach them to the ocean floor, and pull against them to move.

CHORDATES: IMPROVING THE
SKELETON
• Chordates are deuterostomes that belong to the phylum Chordata.
• They exhibit a truly internal endoskeleton with muscles attached to an internal rod, called a notochord .
• This innovation opened the door to large body sizes not possible in earlier animal forms.

CHORDATES: IMPROVING THE
SKELETON
• The approximately 56,000 species of
• Notochord
• Nerve cord
• Pharyngeal pouches
• Postanal tail
• All chordates have all four of these characteristics at some time in their lives.

CHORDATES: IMPROVING THE
SKELETON
• Not all chordates are vertebrates.
• Tunicates and lancelets are chordates.

CHORDATES: IMPROVING THE
SKELETON
• Vertebrate chordates differ from tunicates and lancelets in two important respects.
• Vertebrates have a backbone that replaces the role of the notochord.
• Vertebrates have a distinct and welldifferentiated head.