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Chapter 31: Animals: Part II

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Echinoderms

Echinoderms and chordates are deuterostomes.

In deuterostomes, the second embryonic opening becomes the mouth and a coelom forms by outpocketing of the primitive gut making these animals enterocoelomates.

A dipleurula larva is found among some.

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Characteristics of Echinoderms

Echinoderms are a diverse group of marine animals; there are no terrestrial echinoderms.

They have an endoskeleton consisting of spine-bearing, calcium-rich plates.

Echinoderms are often radially symmetrical, although the larva is a free-swimming filter feeder with bilateral symmetry.

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Echinoderm DiversityEchinoderms include:

Sea lilies (class Crinoidea)

Sea cucumbers (class Holothuroidea)

Brittle stars (class Ophiuroidea)

Sea urchins and sand dollars (class Echinoidea)

Sea stars (class Asteroidea)

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Echinoderm diversity

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Sea StarsSea stars are an example of echinoderms

and possess tiny skin gills, a central nerve ring with branches, and a water vascular system for locomotion.

Water enters this system through the sieve plate, passes into a ring canal, then into ampullae, and into tube feet; expansion and contraction of tube feet move the sea star along.

Each of the five arms contains branches from the nervous, digestive, and reproductive systems.

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When a sea star eats a bivalve, it everts its cardiac stomach into the bivalve and secretes enzymes; partially digested food is taken into the sea star.

Echinoderms do not have a respiratory, circulatory, or excretory system.

The water vascular system carries out these functions.

Sea stars reproduce both sexually, and asexually by fragmentation.

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Sea star anatomy and behavior

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Chordates

Chordates (tunicates, lancelets, and vertebrates) have:

a suuporting notochord,

a dorsal hollow nerve cord,

pharyngeal pouches, and a

post-anal tail at one time during their development.

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Chordate characteristics

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Evolution of ChordatesThe lancelets and tunicates are

invertebrate chordates.Vertebrates include the fishes,

amphibians, reptiles, birds, and mammals.

Cartilaginous fishes were the first to have jaws; amphibians evolved legs and invaded land.

Reptiles, birds, and mammals have means of reproduction suitable to land.

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Evolutionary tree of chordates

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Invertebrate ChordatesLancelets and tunicates are the invertebrate

chordates. Lancelets are small animals found in

shallow water along the coasts; they filter feed on microscopic organisms.

Tunicates (sea squirts) live on the ocean floor and filter water entering the animal through an incurrent siphon.

Adult tunicates lack chordate characteristics except gill slits, but adult lancelets retain the four chordate characteristics.

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Habitat and anatomy of a lancelet, Brachiostoma

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Anatomy of a tunicate, Halocynthia

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Vertebrates

At some time during their lives, all vertebrates have the four chordate characteristics.

The notochord is replaced by the vertebral column; this endoskeleton demonstrates segmentation.

The internal organs are well developed and cephalization places complex sense organs at the head.

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Vertebrates are distinguished in particular by these features:

Living endoskeletonClosed circulatory system

Paired appendagesEfficient respiration and excretion

High degree of cephalizationThe evolution of jaws allowed some

vertebrates to take up the predatory way of life.

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Milestones in vertebrate evolution

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FishesToday there are three living classes of

fishes: jawless fishes, cartilaginous fishes, and bony fishes – the last two groups have jaws.

Jawless FishesThe first vertebrates were jawless fishes,

today represented by hagfishes and lampreys with no scales or paired fins.

Water moves in and out through gill openings.

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Cartilaginous FishesThe cartilaginous fishes include the

sharks, rays and skates which have skeletons made of cartilage.

Skates and rays are flat fishes that live partly buried in the sand and feed on mussels and clams.

Sharks and rays have a sense of electric currents in water, a lateral line system, and a keen sense of smell; these attributes help detect prey.

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Bony FishesBony fishes have jaws and two pairs of

fins and are the most diverse and numerous of all vertebrates.

Bony fishes include those that are ray-finned (most abundant) and a few that are lobe-finned; some of the lobed-finned fishes have lungs and likely gave rise to amphibians.

A swim bladder may provide buoyancy in ray-finned fishes.

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Most fishes today are ray-finned and have these characteristics:

Bony skeleton and scales

Swim bladder

Two-chambered heart (one atrium, one ventricle)

Paired fins

Jaws

Gills

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Jawed fishes

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Amphibians Amphibians evolved from the lobe-

finned fishes and are tetrapods with two pairs of limbs.

They are represented today by frogs, newts, toads, and salamanders.

Amphibians usually return to the water to reproduce and require moist habitats.

Frog tadpoles metamorphose into terrestrial adults with lungs.

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These features distinguish amphibians:

Usually tetrapods

Mostly metamorphosis

Three-chambered heart (2 atria, one ventricle)

Usually lungs in adults

Smooth, moist skin

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Frog metamorphosis

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ReptilesReptiles include the extinct dinosaurs and

today’s snakes, lizards, turtles, alligators, and crocodiles.

Reptiles have well-developed lungs within a rib cage; they are covered with scales that protect them from desiccation and predators.

Reptiles have internal fertilization and also lay a shelled egg, which contains extraembryonic membranes, including an amnion that allows the embryo to develop on land.

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Features that distinguish reptiles include:Usually tetrapods

Lungs with expandable rib cagesShelled, leathery egg

Dry, scaly skinFishes, amphibians, and reptiles are

ectothermic.Reptiles try to regulate their body

temperature by moving to a warmer or cooler location as needed.

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The tongue as a sense organ

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The reptilian egg allows reproduction on land

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Birds

Birds are characterized by the presence of feathers, which are modified reptilian scales.

Birds lay hard-shelled eggs rather than the leathery eggs of reptiles.

Birds are likely closely related to bipedal dinosaurs, although this is still under study.

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Bird anatomy

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Anatomy and Physiology of BirdsFeatures of birds are related to the ability

to fly.Bird forelimbs are modified as wings.Bones are hollow and laced with air

cavities; the sternum has a keel to which flight muscles attach.

A horny beak replaces teeth.Respiration is efficient due to air sacs.Birds have a four-chambered heart, and

birds are homeothermic.

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Bird circulatory system

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Classification of BirdsThe classification of birds is based on

beak and foot types and to some extent on habitat and behavior.

These features distinguish birds:Feathers

Hard-shelled eggFour-chambered heart

Usually wings for flyingAir sacs

Homeothermic

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Bird beaks

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Mammals Mammals evolved from reptiles and

flourished after the demise of dinosaurs. Mammals have hair that helps them

maintain a constant body temperature.Like birds, mammals have a four-

chambered heart.Internal development in the uterus

shelters the young.Mammary glands allow mammals to

nourish their young.

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MonotremesMonotremes have a cloaca that is a

common area for feces, excretory wastes, and sex cells.

Monotremes lay hard-shelled amniote eggs.

Monotremes are represented by the duckbill platypus and the spiny anteater.

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MarsupialsMarsupials have a pouch in which the

very immature newborn matures.

Inside the pouch, the newborns attach to nipples of mammary glands.

Marsupials are represented by the American opossum, and various Australian animals such as koalas and kangaroos.

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Monotremes and marsupials

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Placental MammalsMost mammals are placental mammals,

which retain the offspring inside a uterus until birth; extraembryonic membranes are present, including the chorion that contributes to the fetal portion of the placenta.

The classification of these mammals is based on methods of obtaining food, and mode of locomotion.

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Mammals are adapted to life on land and can move rapidly.

The brain is enlarged due to the expansion of the cerebral hemispheres.

Internal body temperature is constant.

Mammals have differentiated teeth; the specific size and shape of the teeth may be associated with whether the animal is a herbivore, a carnivore, or an omnivore.

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These features distinguish placental mammals:

Body hair

Differentiated teeth

Infant dependency

Constant internal temperature

Mammary glands

Well-developed brain

Internal development

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Placental mammals

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PrimatesPrimates are mammals adapted to living in

trees; many have an opposable thumb.The snout is shortened, enabling

stereoscopic vision, and cone cells give greater visual acuity.

During the evolution of primates, various groups diverged in a particular sequence.

Prosimians include lemurs, tarsiers, and lorises; anthropoids include monkeys, apes, and humans.

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These traits distinguish primates from other mammals:

Opposable thumb (and sometimes great toe)

Well-developed brain

Nails (not claws)

Single birth

Extended period of parental care

Emphasis on learned behavior

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Human EvolutionThe primate evolutionary tree shows that

all primates share one common ancestor and that the other lines of descent diverged from the human lineage over time.

Humans and apes shared a common ancestor.

Molecular data indicate we are most closely related to the African apes, whose ancestry split from ours about 6 MYA.

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Primate evolutionary tree

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Evolution of Hominids

To be a hominid, a fossil must have an anatomy enabling it to stand erect and walk on two feet (bipedalism).

Ardipithecus ramidus kadabba is a 5.6 to 5.2 million-years-ago (MYA) hominid found in Ethiopia; Ardipithecus ramidus ramidus is a 4.4 MYA hominid and less apelike than the older fossil.

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AustralopithecinesHuman evolution continued in eastern Africa

around 4 MYA with the evolution of the australopithecines, a group that is a direct ancestor to humans.

Raymond Dart discovered Australopithecus africanus in southern Africa in the 1920s; this fossil was the gracile type dated at 2.8 MYA.

A more robust form, (A. robustus), from 2 to 1.5 MYA, had a brain size of 500cc like A. africanus; these hominids may have been bipedal but still had longer forelimbs.

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The most famous australopithecine is “Lucy” or A. afarensis (3.18 MYA) unearthed in eastern Africa, whose brain was small (400 cc) but who walked bipedally.

Since the australopithecines were apelike above the waist but humanlike below the waist, it seems that human characteristics did not all evolve at once.

This type of evolution of various body parts at different rates is referred to as mosaic evolution.

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Australopithecus afarensis

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Australopithecus afarensis, a gracile type, is believed to be ancestral to the robust types found in eastern Africas: A. aethiopicus and A. boisei.

A. boisei had a powerful upper body and the largest molars of any hominid.

These robust types died out, and therefore, it is possible that A. afarensis is ancestral to both A. africanus and Homo.

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Evolution of Early HomoFossils are assigned to Homo if the brain

size is 600 cc or greater, if the jaw and teeth resemble those of humans, and if tool use is evident.

Homo habilis Homo habilis, (handy man) present at

about 2 MYA, is certain to have made crude flake-like stone tools.

Speech areas of the brain enlarged and contributed to the beginning of society and culture.

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Human evolution

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Homo erectusBetween 1.9 and 0.3 MYA Homo erectus,

with a brain capacity of 1,000 cc, a striding gate, and a flatter face, was the first to migrate out of Africa into Asia and Europe about 1 MYA.

Males were about 6 feet tall and females approaching 5 feet, much taller than earlier hominids.

H. erectus was the first hominid to use fire and tools of this time were advanced axes and cleavers.

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Homo erectus

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Evolution of Modern HumansTwo contradicting hypotheses have been

suggested about the origin of modern humans, Homo sapiens, from H. erectus.

The multiregional continuity hypothesis suggests that modern humans originated from H. erectus separately in Asia, Europe, and Africa.

The out-of-Africa hypothesis states that modern humans originated in Africa and, after migrating into Europe and Asia, replaced the archaic Homo species found there.

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Multiregional continuity hypothesis

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Out-of-Africa hypothesis

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NeanderthalsThe Neanderthals lacked a high forehead

and a significant chin and are classified as Homo neanderthalensis.

They had massive brow ridges; their pubic bone was long compared to that of modern humans.

Neanderthals had a brain larger than that of modern humans, and they lived in Europe and Asia during the last Ice Age.

Neanderthals lived in caves, made stone tools, and buried their dead with flowers.

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Neanderthals

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Cro-MagnonsCro-Magnons evolved about 100,000

years ago and were the first humans (Homo sapiens) to have a thoroughly modern appearance.

They made stone tools, including stones attached to wooden handles; they threw spears, enabling them to cooperatively hunt larger animals.

The Cro-Magnon culture included art and beautiful paintings on cave walls.

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Cro-Magnons

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Chapter SummaryBoth echinoderms and chordates are

deuterostomes.In deuterostomes, the second embryonic

opening becomes the mouth; the coelom develops by an outpocketing from the primitive gut.

Echinoderms develop radial symmetry and have a unique water vascular system for locomotion.

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Chordates have a notochord, a dorsal tubular nerve cord, and a post-anal tail; pharyngeal pouches occur sometime during the life of chordates, and in vertebrates, the notochord is replaced by the vertebral column.

There are three groups of fishes.

One group is jawless, but the cartilaginous and bony fishes (ray-finned and lobe-finned) have jaws.

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Frogs and salamanders are amphibians that evolved from lobe-finned fishes; they have limbs as an adaptation for locomotion on land.

The shelled egg of reptiles contains extraembryonic membranes as an adaptation for reproduction on land.

Both birds with feathers, and mammals with hair and mammary glands, evolved from reptiles and are able to maintain a constant body temperature.

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Primates such as prosimians, monkeys, apes, and humans are mammals adapted to living in trees.

Human evolution diverged from ape evolution in Africa about six to seven million years ago.

The australopithecines were the first hominids and were ancestors to humans.

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Homo habilis could make tools; Homo erectus migrated out of Africa.

The Neanderthals (H. neanderthalensis) gave evidence of being culturally advanced, lived in caves, and hunted large mammals; they buried their dead with flowers.

Cro-Magnons are the oldest fossils to be designated Homo sapiens; they made sophisticated tools and were accomplished artists and hunters.