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Chapter 19Evolution of

Animals

Key Innovations Distinguish Invertebrate Groups

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19.1 Animals have distinctive characteristics

Animals are multicellular eukaryotes Chemoheterotrophs that acquire nutrients from an

external source and digest it internally Usually carry on sexual reproduction and begin life only

as a fertilized diploid egg Developmental stages to produce specialized tissues

within organs with specific functions Muscles and nerves characterize animals

Allow animals to perform flexible movements

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Figure 19.1 Developmental stages of a frog

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19.2 Animals most likely have a protistan ancestor

Two hypotheses on origin of animals Multinucleate Hypothesis Animals arose from a ciliated protist in stages

The ciliate would have acquired multiple nuclei, and then it would have become multicellular

Colonial Flagellate Hypothesis Animals descended from an ancestor that resembled a

spherical colony of flagellated cells

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Figure 19.2 The colonial flagellate hypothesis

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Radial and Bilateral Symmetry

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19.3 The traditional evolutionary tree of animals is based on seven key innovations (figure 19.3A)

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Animal Body Cavities

Figure 19.3B Types of body cavity

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APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES

19.4 Molecular data suggest a new evolutionary tree for animals

In traditional tree, protostomes are restricted to three phyla, which have a coelom Arthropods, Annelids, and Molluscs

Evolutionary tree based on molecular data suggests many more animal phyla should be designated protostomes because their rRNA sequences are so similar Segmentation doesn’t play a defining role in the

evolutionary tree based on molecular data

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Figure 19.4A Proposed new evolutionary tree

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Figure 19.4B Roundworms and arthropods are molting animals

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19.5 Some animal groups are invertebrates and some are vertebrates

For convenience, the animal phyla have been divided into Invertebrates - those that do not have an

endoskeleton of cartilage and bone Vertebrates - those that do have an endoskeleton

Animals evolved in the sea and most animals still live in the water Among the invertebrates only the molluscs, annelids,

and arthropods have terrestrial representatives19-13

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19.6 Sponges are multicellular invertebrates

Only animal without true tissue, organized at cellular level Body of Sponges - phylum Porifera because their bodies

are perforated by pores Filter feeder, also called a suspension feeder, because it filters

suspended particles from water Endoskeleton

Have fibers of spongin, a modified form of collagen Typically have an endoskeleton also with spicules, small, needle-

shaped structures with one to six rays Reproduction

Reproduce asexually by budding Reproduce sexually as egg and sperm are released into central

cavity Zygote develops into a flagellated larva that may swim to a new location

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Figure 19.6 Sponge anatomy

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19.7 Cnidarians have true tissues

Cnidarians (phylum Cnidaria) - an ancient group of invertebrates Most in the sea, but a few freshwater species

Radially symmetrical and capture their prey with a ring of tentacles that have specialized stinging cells, cnidocytes Each cnidocyte has a capsule called a nematocyst,

containing a long, spirally coiled, hollow thread Two basic body forms

Polyp - mouth is directed upward from the substrate Medusa - mouth is directed downward

Cnidarians - a sac body plan with only one opening

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Figure 19.7A Cnidarian diversity

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Figure 19.7B Anatomy of Hydra, a polyp

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19.8 Free-living flatworms have bilateral symmetry

Flatworms (phylum Platyhelminthes) First phylum with bilateral symmetry

Have three germ layers Ectoderm from which body wall develops Endoderm from which digestive cavity develops Mesoderm which contributes to organ formation

Have no coelum and are called acoelomates Planarians have several body systems

Digestive System - pharynx leads to gastrovascular cavity Excretory System - a series of interconnecting canals Reproductive System - Hermaphrodites (both male and female

sex organs) and perform cross-fertilization Nervous System - brain and two lateral nerve cords are joined

by cross-branches called transverse nerves 19-20

Figure 19.8 Planarian anatomy

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19.9 Some flatworms are parasitic

Tapeworms Endoparasites (internal parasites) of various vertebrates,

including humans Vary in length from a few millimeters to 20 meters

Tough body covering resistant to the host’s digestive juices Scolex bears hooks and suckers for attachment to the intestinal

wall of the host Flukes

Endoparasites of various vertebrates Anterior end of the animal has an oral sucker and at least one

other sucker used for attachment to the host Nearly 800,000 persons die each year from an infection called

schistosomiasis

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Figure 19.9A Tapeworm (Taenia solium) anatomy and life cycle

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Figure 19.9B Sexual portion of blood fluke (Schistosoma spp.) life cycle

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19.10 Roundworms have a pseudocoelom and a

complete digestive tract Roundworms (phylum Nematoda) possess two anatomic

features not previously seen body cavity - pseudocoelom and is incompletely lined with

mesoderm complete digestive tract - it has both a mouth and an anus

Nonsegmented, meaning that they have a smooth outside body wall

Ascaris Humans are infected with a roundworm called Ascaris when eggs

enter the body via uncooked vegetables Other Roundworm Parasites

Trichinosis - serious human infection acquired when humans eat meat that contains encysted larvae

Elephantiasis is caused by a roundworm called a filarial worm, which utilizes mosquitoes as a secondary host

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Figure 19.10A Ascaris Figure 19.10b Encysted Trichinella larva

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Figure 19.10C Elephantiasis

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19.11 A coelom gives complex animal groups certain advantages

Coelom - body cavity completely lined by mesoderm Two groups

Protostomes - Molluscs, annelids, and arthropods Deuterostomes - echinoderms and chordates

Two major events can be used to distinguish protostomes from deuterostomes Blastopore - protostomes: mouth appears near blastopore;

deuterostomes: anus appears near blastopore Coelom formation - protostomes: splitting produces the coelom;

deuterostomes: the coelom arises as a pair of mesodermal pouches from the gut wall

Advantages of a Coelom Body movements are freer because outer wall can move

independently of the enclosed organs Ample space of a coelom allows complex organs and organ systems

to develop19-28

Figure 19.11 Protostomes compared to deuterostomes

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19.12 Molluscs have a three-part body plan

All molluscs (phylum Mollusca) have a body composed of at least three distinct parts The foot - strong, muscular portion used for locomotion Visceral Mass - soft-bodied portion that contains internal organs The mantle - covering that envelops the visceral mass

The mantle may secrete an exoskeleton called a shell Gastropods (meaning stomach-footed) including snails and

nudibranchs Animal moves by muscle contractions that pass along its ventrally

flattened foot Cephalopods (meaning head-footed) including octopuses, squids,

and nautiluses The foot has evolved into tentacles about the head

Bivalves (shells have two part) including clams, oysters, scallops, and mussels The clam is a filter feeder and food particles and water enter the mantle

cavity by way of a siphon19-30

Figure 19.12A Body plan of a typical mollusc

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Figure 19.12B Three groups of molluscs

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19.13 Annelids are the segmented worms

Annelids (phylum Annelida) are segmented, as can be seen externally by the rings that encircle the body Partitions called septa divide the well-developed, fluid-filled

coelom, which is used as a hydrostatic skeleton Excretory system consists of nephridia, tubules that collect

waste material and excrete it through an opening in the body wall Oligochaetes

Earthworm is an oligochaete because it has few setae, bristles that anchor the worm

Polychaetes Most annelids are polychaetes (having many setae per segment)

that live in marine environments Leeches

Have no setae, but have the same body plan as other annelids Blood suckers that are able to keep blood flowing and prevent

clotting19-33

Figure 19.13A Earthworm anatomy

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Figure 19.13B Other annelids

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19.14 Arthropods have jointed appendages

Arthropods (phylum Arthropoda) are extremely diverse More than one million species have been discovered

Six characteristics Jointed appendages Exoskeleton Segmentation Well-developed nervous system Variety of respiratory organs Reduced competition through metamorphosis

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Figure 19.14A Exoskeleton and jointed appendages of a crayfish, an arthropod

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Figure 19.14B Monarch butterfly metamorphosis

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19.15 Well-known arthropods other than insects

Crustaceans - name derived from their hard, crusty exoskeleton Largely marine arthropods that include crabs, barnacles,

shrimps, and crayfish Head usually bears a pair of compound eyes and five

pairs of appendages

Arachnids include spiders, scorpions, ticks, mites, and harvestmen Spiders have a narrow waist that separates the

cephalothorax, with four pairs of legs, from the abdomen19-39

Figure 19.15A Crustacean diversity

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Figure 19.15B Centipede and millipede

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Figure 19.15C Spider and relatives

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19.16 Insects, the largest group of arthropods, are adapted

to living on land

Insects - so numerous (>one million species) and diverse that the study of this one group is a major specialty in biology called entomology

Adapted to a life on land Body is divided into head, thorax, and abdomen Mouthparts adapted to each species’ way of life Wings enhance an insect’s ability to survive

way of escaping enemies, finding food, facilitating mating, and dispersing offspring

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Figure 19.16 Insect diversity

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19.17 Echinoderms are radially symmetrical as adults

Echinoderms (phylum Echinodermata) lack chordate features, but are related to because they are deuterostomes Radially, not bilaterally, symmetrical as adults

Their larva is a bilaterally symmetrical filter feeder Adult echinoderms do not have a head, brain, or segmentation

Their nervous system consists of nerves in a ring around the mouth extending outward radially

Locomotion by a water vascular system that pumps water into many tube feet, expanding them

No complex respiratory, excretory, or circulatory system Fluids within the coelomic cavity and the water vascular system

carry out many of these functions In ecosystems, most feed on organic matter in the sea or

substratum Sea stars prey upon crustaceans, molluscs, and other invertebrates

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Figure 19.17 Echinoderm structure and diversity

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Further Innovations Allowed Vertebrates to Invadethe Land Environment

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19.18 Four features characterize chordates

Dorsal supporting rod (notochord) extends the length of the body

Dorsal tubular nerve cord contains a canal filled with fluid

Pharyngeal pouches only during embryonic development in most vertebrates

A postanal tail extends beyond the anus

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Figure 19.18 The four chordate characteristics

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19.19 Invertebrate chordates have a notochord as adults

A few of the invertebrate chordates never replace the notochord with the vertebrae Tunicates (subphylum Urochordata) live on the

ocean floor as filter feeders Larva is bilaterally symmetrical and has the four chordate

characteristics Metamorphosis produces the sessile adult

Lancelets (subphylum Cephalochordata) marine chordates only a few centimeters long Lancelets retain the four chordate characteristics as adults

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Figure 19.19 The invertebrate chordates

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19.20 The evolutionary tree of vertebrates is based on

five key features

Vertebrates Fishes, amphibians, reptiles, birds, and mammals Vertebrae are most obvious feature Vertebral column is flexible because vertebrae are

separated by disks, which cushion the vertebrae The soft center of a disk presses on the spinal cord

Derived Characters Among Vertebrates Jaws, Lungs, Jointed Limbs, and Amniotic Eggs

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Figure 19.20 Evolutionary tree of the chordates

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19.21 Jaws and lungs evolved among the fishes

Jawless Fishes (Class Agnatha) Cylindrical and up to a meter long Smooth, scaleless skin, no jaws or paired fins

Cartilaginous Fishes (Class Chondrichthyes) includes sharks, the rays, and the skates Skeletons of cartilage, instead of bone Shark senses

Able to sense electric currents in water Lateral line system senses pressure waves caused by fish Keen sense of smell

Bony Fishes (Class Osteichthyes) most numerous and diverse of all vertebrates Ray-finned fishes - use their fins to balance and propel body

Have a swim bladder, which usually serves as a buoyancy organ Bony scales that protect body but do not prevent water loss

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Figure 19.21A Evolution of jaws

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Figure 19.21B Diversity of fishes

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Figure 19.21C This transitional form links the lobes of the lobe-finned fishes to the limbs of ancestral amphibians

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19.22 Amphibians are tetrapods that can move on land

Amphibians (class Amphibia) means living on both land and in the water, represented by frogs, toads, newts, and salamanders

Characteristics Adults have small lungs - air enters the mouth by way

of nostrils Respiration is supplemented by gas exchange

through the smooth, moist, skin

Most members lead an amphibious life Larval stage lives in water, and adult stage is on land

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Figure 19.22 Frogs and salamanders are well-known amphibians

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19.23 Reptiles have an amniotic egg and can reproduce on land

Reptiles (class Reptilia) diversified and most abundant between 245 and 66 MYA

The reptiles living today are mainly alligators, crocodiles, turtles, snakes, lizards, and tuataras

Body is covered with hard, keratinized scales, which protect animal from desiccation and from predators

Fertilization is internal, and the female lays leathery, flexible, shelled eggs Amniotic egg made development on land possible and

eliminated the need for a swimming larval stage Fishes, amphibians, and reptiles are ectotherms

Body temperature matches the temperature of their environment

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Amniotic Egg

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Figure 19.23 Reptilian diversity

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19.24 Birds have feathers and are endotherms

Birds (class Aves) are characterized by the presence of feathers Lay a hard-shelled amniotic egg

Data suggests birds are related to bipedal dinosaurs and should be classified as such

Birds are adapted to fly Forelimbs are modified as wings Hollow, light bones Horny beak has replaced jaws with teeth

Birds are endotherms and generate internal heat May be associated with efficient nervous, respiratory, and

circulatory systems Seasonal migration of many species over very long distances

Navigate by day and night, whether it’s sunny or cloudy, by using the sun and stars and even the Earth’s magnetic field to guide them

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Figure 19.24A Bird flight

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Figure 19.24B Types of bird beaks

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19.25 Mammals have hair and mammary glands

Mammals (class Mammalia) evolved during Mesozoic from reptiles called therapsids

Two chief characteristics Hair: Mammals are endotherms, and hair aids

temperature control Milk-producing mammary glands: enable females to

feed (nurse) their young without leaving them to find food

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Monotremes and Marsupials

Monotremes - mammals that have a cloaca, a terminal region of the digestive tract serving as a common chamber for feces, excretory wastes, and sex cells Also lay hard-shelled amniotic eggs

Marsupials - begin their development inside the female’s body, but they are born in a very immature condition Newborns crawl up into a pouch on their mother’s

abdomen

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Figure 19.25A Monotremes and marsupials

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Placental Mammals

Placental mammals - extraembryonic membranes of the reptilian egg are modified for internal development within the uterus of the female Chorion contributes to the fetal portion of the placenta, while

a part of the uterine wall contributes to the maternal portion Nutrients, oxygen, and waste exchanged between fetal and

maternal blood Distinguished by their mode of locomotion

Bats have membranous wings supported by digits Horses have long, hoofed legs; and whales have paddlelike

forelimbs Distinguished by the way of obtaining food

Mice have continuously growing incisors Horses have large, grinding molars Dogs have long canine teeth

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Figure 19.25B Placental mammals

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES

19.26 Many vertebrates provide medical treatments for humans

Hundreds of pharmaceutical products come from vertebrates Animals that produce poisons and toxins give us

medicines that benefit us Some produce proteins similar to human proteins to

be used in medical treatment Powerful applications of genetic engineering

found in development of drugs and therapies for human diseases Xenotransplantation, transplantation of vertebrate

tissues and organs into human beings Use of transgenic vertebrates for medical purposes

does raise health and ethical concerns 19-71

Figure 19.26 Vertebrates used for medical purposes

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Connecting the Concepts:Chapter 19

As terrestrial mammals, humans might assume that terrestrial species are more successful than aquatic ones If not for the myriad types of terrestrial insects, there would be

more aquatic species than terrestrial ones on Earth Adaptative radiation of mammals has taken place on

land, and this might seem impressive to some Actually, the number of mammalian species (4,800) is small

compared to the molluscs (110,000 species) Size and complexity of the brain is also sometimes cited

as a criterion by which vertebrates are more successful than other living things This characteristic has been linked to others that make an

animal prone to extinction Long life span, slow to mature, have few offspring, expend much

energy caring for their offspring, and tend to become extinct if their normal way of life is destroyed

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