Campbell Biology Chapter 32 Review: An Overview of Animal Diversity

32.1 What all animals have in common

pp. 654–656

On the AP exam? Background

The current course won't ask what defines an animal or make you name the stages of an embryo. What carries over: genes switched on in sequence guide development (Topic 6.6), animal eggs and sperm come straight from meiosis (Topic 5.1), and animal cells have no wall (Topic 2.4).

In the course: Topic 6.6 Gene Expression and Cell Specialization, Topic 5.1 Meiosis, Topic 2.4 Membrane Permeability (notes, videos and more questions)

Key points

Animals are many-celled eukaryotes that can't make their own food. Unlike plants, they get organic molecules by eating. Unlike fungi, which digest food outside themselves and soak up the products, animals take food in first and break it down inside the body.
Animal cells have no cell wall. A mesh of proteins outside the plasma membrane, mostly collagen, supports the cells and ties them together instead.
Most animals have two kinds of cells that no other many-celled group has: nerve cells that carry signals and muscle cells that contract. Organized into tissues, they let animals move and react fast. Sponges are an exception and have neither.
In most animals the diploid stage is the main one. Meiosis makes eggs and sperm directly, with no many-celled haploid stage in between, which sets animals apart from plants and many fungi.
Early development follows a shared path. The zygote divides again and again without growing (cleavage) into a ball of cells, the blastula. Then cells move inward (gastrulation), giving a layered gastrula whose inner pouch becomes the gut.
Many animals pass through a larva that looks, eats and often lives differently from the adult, then reshape into a juvenile by metamorphosis. Others, including people, develop straight into a small version of the adult.
Hox genes code for transcription factors that tell cells where they sit along the head-to-tail axis by switching other genes on and off. They've changed very little over hundreds of millions of years and are shared by flies, worms and people. Sponges lack them, though they have other homeobox genes.

Key terms (15)

heterotroph: An organism that can't build its own organic molecules and has to get them from other organisms or their remains. Every animal is a heterotroph.
ingestion: Taking food into the body, for example by swallowing, before digesting it. It's how animals feed, unlike fungi, which absorb food that's already been broken down outside them.
extracellular matrix: The web of proteins and other molecules that animal cells release around themselves. It supports cells and links them, doing the job a cell wall does in plants.
collagen: A tough, rope-like protein that is the most plentiful part of the animal extracellular matrix. It's a big reason your tendons and skin are strong.
tissue: A group of similar cells working together on one job, such as muscle tissue for movement or nervous tissue for carrying signals.
zygote: The single diploid cell formed when a sperm fuses with an egg. Every cell of the future animal comes from it.
cleavage: The fast rounds of mitosis right after fertilization. Cells don't grow between divisions, so they get smaller each round.
blastula: The early embryo that cleavage produces, often a hollow ball of cells.
blastocoel: The fluid-filled space inside a hollow blastula.
gastrulation: The stage when some cells of the blastula move inside, rearranging the embryo into two or three layers.
archenteron: The pouch formed during gastrulation that becomes the gut.
blastopore: The opening of the archenteron to the outside, where cells moved inward. Depending on the animal, it becomes the mouth or the anus.
larva: An immature stage of an animal's life with a body form unlike the adult's. It can't breed yet, and it often feeds on other food or in another habitat.
metamorphosis: The body overhaul that turns a larva into a young animal with the adult's body form, though it still has to mature before it can breed.
Hox genes: A family of homeobox genes (genes with a shared stretch that codes for a DNA-binding part of a protein). Hox proteins mark positions along the head-to-tail axis and have changed little over time.

Check yourself: 32.1 What all animals have in common

4 questions on 32.1 What all animals have in common. Pick an answer to see if you got it, and why.

Question 1 of 4

Divers collect two unknown many-celled eukaryotes from a sunken log. Organism 1 releases enzymes onto the wood and takes up the dissolved products across its cell surfaces. Organism 2 draws bits of wood into an internal cavity, where its cells release enzymes and absorb the products. Neither has chloroplasts. Which observation is the best evidence that Organism 2 is an animal and Organism 1 is not?

Question 2 of 4

An embryo goes from a single-celled zygote to 64 cells through cleavage, with every cell dividing once in each round. Which description of the 64-cell embryo is most accurate?

Question 3 of 4

A student looks at a slice through an early sea star embryo. It is shaped like a cup with a double wall: an outer sheet of cells and an inner sheet that lines a pouch, which opens to the outside through a single hole. What stage is this, and what will the pouch become?

Question 4 of 4

How does the way a deer makes its gametes differ from the way a moss makes its gametes?

0 of 4 answered

32.2 Half a billion years of animal history

pp. 656–658

On the AP exam? Background

The exam won't ask about eras, dates or Ediacaran fossils. It does use fossils and molecular clocks as evidence (Topics 7.6 and 7.9), and the Cambrian explosion and the burst of mammals after the dinosaurs make good examples of rapid speciation and adaptive radiation (Topic 7.10).

In the course: Topic 7.6 Evidence of Evolution, Topic 7.9 Phylogeny, Topic 7.10 Speciation (notes, videos and more questions)

Key points

Molecular clocks put the last common ancestor of living animals roughly 800 to 650 million years ago, well before the oldest animal fossils most scientists accept. Early animals were likely small and soft, so they seldom fossilized.
Choanoflagellates, protists that live alone or in colonies, are animals' closest living relatives. Their collared feeding cells look almost exactly like a sponge's, DNA puts them next to animals, and they carry genes for cell-sticking and signaling proteins once thought to be animal-only.
The first large animal fossils, the soft-bodied Ediacaran biota, date from about 575 to 539 million years ago. Some look like sponges or cnidarians and many are hard to place, but chemical traces show at least some were animals.
In the Cambrian explosion, roughly 535 to 520 million years ago, the earliest fossils of roughly half of today's animal phyla appear, including arthropods, chordates and echinoderms, along with the first hard skeletons. (Newer dating starts the Cambrian about 539 million years ago, not 542.)
Three ideas, which can all be partly right, try to explain the Cambrian explosion: an arms race between new predators and better-defended prey, more oxygen to fuel bigger and more active bodies, and new developmental genes such as Hox genes that opened up new body forms.
In the Paleozoic, arthropods and then vertebrates (by about 365 million years ago) moved onto land. The Mesozoic brought dinosaurs, flying reptiles, the first birds and small early mammals, and reefs built by modern-type corals; late in the era, insects and flowering plants diversified together.
A mass extinction 66 million years ago ended the large dinosaurs and the big marine reptiles. Mammals then branched into many new forms to fill the empty niches, an adaptive radiation that went on as the climate cooled through the Cenozoic.

Key terms (11)

molecular clock: A way to estimate when two lineages split, by counting the DNA or protein differences between them and dividing by how fast differences build up.
choanoflagellate: A protist with one flagellum ringed by a collar of tiny projections, living alone or in colonies. It's the closest living relative of animals.
collar cell: A sponge cell (also called a choanocyte) whose flagellum sits inside a collar that strains food from the water. It looks almost the same as a choanoflagellate.
Ediacaran biota: Soft-bodied organisms preserved in rocks from about 575 to 539 million years ago, including some of the earliest known animals.
Cambrian explosion: A burst of animal evolution early in the Cambrian period, when the first fossils of many modern animal groups and the first hard shells and skeletons show up.
Paleozoic era: The era from about 539 to 252 million years ago, when animals spread through the oceans and then onto land.
Mesozoic era: The era from about 252 to 66 million years ago, often called the age of dinosaurs. The first mammals and birds appeared during it.
Cenozoic era: The era from 66 million years ago to today, when mammals and birds filled many roles left open by the dinosaurs.
mass extinction: A geologically short stretch of time when a large share of Earth's species die out, as at the end of the Permian and the end of the Cretaceous.
adaptive radiation: When one group quickly branches into many species suited to different ways of life, often after open niches appear.
tetrapod: A vertebrate with four limbs, or one descended from four-limbed ancestors: amphibians, reptiles (birds included) and mammals.

Check yourself: 32.2 Half a billion years of animal history

4 questions on 32.2 Half a billion years of animal history. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

A gene builds up differences between two diverging lineages at a steady 0.16 differences per million years, counting the changes in both lineages together. A comb jelly and a sea star differ at 112 positions in this gene. According to this clock, about how long ago did their lineages split?

Question 2 of 4

Molecular clocks place the origin of animals at least 100 million years before the oldest animal fossils that most paleontologists accept. Which explanation for this gap is most reasonable?

Question 3 of 4

A researcher compares the amino acid sequence of an enzyme from a sponge with the same enzyme in four other eukaryotes (invented data): Organism | Identity with the sponge enzyme Choanoflagellate | 61% Yeast (a fungus) | 44% Green alga | 37% Amoeba | 35% Which conclusion is best supported?

Question 4 of 4

Which finding from early Cambrian rocks would most directly support the hypothesis that an arms race between predators and prey drove the Cambrian explosion?

0 of 4 answered

32.3 Comparing animal body plans

pp. 658–661

On the AP exam? Not tested

Symmetry, germ layers, body cavities and protostome versus deuterostome development aren't in the current course. The idea that a shared body plan doesn't always mean shared ancestry is useful when you read trees in Topic 7.9.

In the course: Topic 7.9 Phylogeny, Topic 6.6 Gene Expression and Cell Specialization (notes, videos and more questions)

Key points

A body plan is the set of body and developmental features that fit together in an animal. Body plans make animals easy to compare, but a similar plan can evolve more than once and features can be lost, so a plan doesn't always reveal ancestry.
Most sponges have no symmetry. A radial animal is arranged around a central axis, with a top and a bottom but no front, back, left or right, which suits living fixed in place or drifting. A bilateral animal has left and right sides, a back (dorsal) and belly (ventral) side, and a head (anterior) and tail (posterior) end, which suits moving forward.
Many bilateral animals show cephalization: sense organs and a brain gathered at the front end, the part that meets new surroundings first.
Gastrulation sorts the embryo into germ layers. Ectoderm makes the outer covering and, in many groups, the nervous system. Endoderm lines the gut and forms organs such as a vertebrate's liver. Mesoderm makes muscle and most organs in between. Jellies and their relatives are diploblastic (two layers); bilateral animals are triploblastic (three).
The space between the gut and the body wall can be a coelom, lined by mesoderm all around, or a pseudocoelom, lined by mesoderm only on the body-wall side. Acoelomates have no cavity. A cavity cushions organs, lets them move on their own, and in soft animals works as a fluid skeleton.
Coelomates and pseudocoelomates are grades, groups that share a body plan, not clades. DNA shows that these cavities were gained or lost several separate times.
Protostome and deuterostome development tend to differ in three ways: spiral, determinate cleavage versus radial, indeterminate cleavage; a coelom made by splitting solid mesoderm versus one made from pouches off the gut; and a blastopore that becomes the mouth versus the anus. Plenty of animals break these patterns.

Key terms (15)

body plan: The overall layout of an animal and the way it develops, such as its symmetry, number of tissue layers and kind of body cavity.
radial symmetry: A body arranged evenly around a central axis, like a wheel or a pie, with a top and bottom but no left or right side.
bilateral symmetry: A body with matching left and right halves, plus a front end, a rear end, a back side and a belly side.
cephalization: The clustering of sense organs and nerve cells into a head at an animal's front end.
ectoderm: The outer germ layer of an embryo (germ layers are the cell layers all tissues come from). It forms the skin or outer covering and, in many animals, the nervous system.
endoderm: The innermost germ layer. It lines the gut and forms organs that grow out from it, like the pancreas and liver in vertebrates.
mesoderm: The middle germ layer, well developed in bilateral animals. It makes muscle, blood, a vertebrate's bones and most organs between the gut and the skin.
diploblastic and triploblastic: Diploblastic animals, like jellies, develop from two germ layers. Triploblastic animals, like worms, insects and you, develop from three.
coelom: A fluid-filled body cavity between the gut and the body wall, lined on every side by tissue that comes from mesoderm.
pseudocoelom: A body cavity with mesoderm on the body-wall side but endoderm on the gut side. The "pseudo" (false) is misleading: it does the same jobs a coelom does.
acoelomate: An animal with three germ layers but no body cavity; tissue fills the space between its gut and its skin.
grade: A group of organisms that share a body plan or level of complexity, whether or not they trace back to one ancestor. Compare it with a clade.
determinate and indeterminate cleavage: In determinate cleavage, each early embryo cell's fate is set right away. In indeterminate cleavage, an early cell split off from the rest can still grow into a whole embryo.
protostome development: A pattern in which cleavage is spiral, the coelom forms by splitting solid mesoderm, and the blastopore usually becomes the mouth.
deuterostome development: A pattern in which cleavage is radial, the coelom forms from pouches off the gut, and the blastopore usually becomes the anus, with the mouth forming later.

Check yourself: 32.3 Comparing animal body plans

4 questions on 32.3 Comparing animal body plans. Pick an answer to see if you got it, and why.

Question 1 of 4

A newly described marine animal drifts in open water. Its body is a soft bell with identical groups of tentacles spaced evenly around the rim, so it looks the same from every side. Which feature is it least likely to have?

Question 2 of 4

In a chick embryo, researchers mark the cells of one germ layer with a dye that is passed on to all their descendants. Days later, the dye shows up in the lining of the small intestine and in the pancreas, but not in the skin, the brain or the leg muscles. Which germ layer was marked?

Question 3 of 4

A cross section of a small, unsegmented worm shows, from the outside in: an outer covering, a layer of muscle that came from mesoderm, a fluid-filled space, and a gut wall made only of cells from endoderm, with no mesoderm around it. How should its body cavity be classified?

Question 4 of 4

A peanut worm burrows by squeezing the muscles of its body wall against the fluid in its coelom, which pushes its front end forward into the sand. If a wound let much of that fluid leak out, what would most likely happen?

0 of 4 answered

32.4 Redrawing the animal family tree with DNA

pp. 662–664

On the AP exam? Background

The exam won't ask you to name animal phyla or clades like Ecdysozoa, but Topic 7.9 tests the skills used here: reading a tree, using shared derived characters, and treating every tree as a hypothesis that new DNA data can change.

In the course: Topic 7.9 Phylogeny, Topic 7.6 Evidence of Evolution, Topic 7.7 Common Ancestry (notes, videos and more questions)

Key points

Zoologists recognize about 35 animal phyla. How they're related is still being worked out, and every tree is a hypothesis that new evidence can revise.
Older trees leaned on body form and embryo development. Today's trees mostly compare DNA from many genes or whole genomes, checked against fossils, with the same goal: finding clades from shared derived characters.
Trees built both ways agree that animals form one clade, that Bilateria (animals with bilateral symmetry and three germ layers) is a clade holding most phyla, and that chordates and echinoderms belong together in Deuterostomia. The Cambrian explosion was mostly a burst of bilaterians.
Older trees made sponges the first branch of animals. Recent genome studies, including ones comparing the order of genes along chromosomes, point to comb jellies instead. If that holds, nerve and muscle cells either evolved twice or were lost in sponges. The question isn't fully settled.
DNA trees split the protostomes into Ecdysozoa and Lophotrochozoa. Older trees grouped arthropods with annelids because both have segments, but DNA puts them in different clades, so segments arose more than once or were lost along the way.
Ecdysozoans, such as arthropods and nematodes, shed their outer covering to grow. Lophotrochozoans, such as molluscs, annelids and flatworms, are named for two features some of them have: the lophophore, a ring of ciliated feeding tentacles, and the trochophore larva.
Acoels, small worms once lumped with flatworms, were moved out by DNA. Many studies put them near the base of Bilateria, hinting that the first bilaterians were simple, with a one-opening gut; other studies place them near the deuterostomes.

Key terms (14)

phylum: A large group of related organisms, one level below kingdom, such as arthropods or chordates. Animals are split into about 35 phyla.
Metazoa: The formal name for the animal clade: all animals, living and extinct, descended from one common ancestor.
Bilateria: The clade of animals with bilateral symmetry and three germ layers. It holds most animal phyla, from flatworms to humans.
Deuterostomia: A clade of bilaterians that includes chordates, like you, and echinoderms, like sea stars.
Protostomia: The clade holding the other main branch of bilaterians, which DNA splits into ecdysozoans and lophotrochozoans.
Ecdysozoa: A clade of protostomes, including arthropods and nematodes, that grow by shedding their outer covering.
ecdysis: Molting: shedding a stiff outer covering so the animal can grow, then making a bigger one.
exoskeleton: A hard covering on the outside of an animal's body that supports and protects it, like the shell of a beetle.
Lophotrochozoa: A clade of protostomes recognized mainly from DNA, including molluscs, annelids and flatworms.
lophophore: A ring of tentacles fringed with beating cilia that some lophotrochozoans use to sweep food particles toward the mouth.
trochophore larva: A tiny swimming larva with a band of cilia around its middle, found in many molluscs and annelids.
Acoela: Small, simple worms with a one-opening gut, once classed as flatworms. DNA shows they belong on a different branch.
sister group: The lineage that shares the most recent common ancestor with a given group; the two are each other's closest relatives on a tree.
basal taxon: A lineage that branches off near the base of a tree, early in a group's history.

Check yourself: 32.4 Redrawing the animal family tree with DNA

4 questions on 32.4 Redrawing the animal family tree with DNA. Pick an answer to see if you got it, and why.

Question 1 of 4

A molecular tree of bilaterians shows two main branches: Deuterostomia (including sea stars and humans) and Protostomia. Protostomia then splits into Ecdysozoa (including insects and nematodes) and Lophotrochozoa (including snails and earthworms). Based on this tree, which pair of animals shares the most recent common ancestor?

Question 2 of 4

Older trees grouped annelids with arthropods because both have bodies built from repeated segments. Large DNA data sets place annelids in Lophotrochozoa and arthropods in Ecdysozoa, each among close relatives that lack segments. If the DNA trees are right, what is the best interpretation of segmentation?

Question 3 of 4

A researcher measures a newly discovered, millimeter-sized animal in a lab culture every day (invented data): Day | Body length (mm) | Shed outer covering found? 1 | 0.40 | No 2 | 0.40 | No 3 | 0.55 | Yes 4 | 0.55 | No 5 | 0.55 | No 6 | 0.75 | Yes Which conclusion is best supported?

Question 4 of 4

Comb jellies have nerve cells and muscle cells, but sponges have neither. Suppose recent genome studies are right that comb jellies, not sponges, are the sister group to all other animals. Which conclusion follows?

0 of 4 answered