Campbell Biology Chapter 33 Review: An Introduction to Invertebrates

33.1 Sponges: animals without true tissues

pp. 670–671

On the AP exam? Not tested

Sponges aren't in the current course. Use them to practice tested ideas: why a thin body can rely on diffusion (Topic 2.2), taking in food by endocytosis (Topic 2.5) and reading what DNA says about who's related to whom (Topic 7.9).

In the course: Topic 2.2 Cell Size, Topic 2.5 Membrane Transport, Topic 5.2 Meiosis and Genetic Diversity, Topic 6.6 Gene Expression and Cell Specialization, Topic 7.9 Phylogeny (notes, videos and more questions)

Key points

Sponges (phylum Porifera) don't move as adults. They strain tiny bits of food, mostly bacteria, out of the water that flows through their bodies. Most live in the sea, though a handful live in lakes and rivers.
Water comes in through many tiny pores, collects in a central chamber called the spongocoel, and leaves through a bigger hole, the osculum. Large sponges have folded walls, branching channels and several oscula.
A sponge has several kinds of cells, but they aren't organized into true tissues: sheets of cells sealed off from each other that work as a unit. That's one reason sponges are seen as an early-branching (basal) animal lineage.
Collar cells, or choanocytes, line the inside. Their beating flagella keep water moving, and a ring of tiny projections around each flagellum catches food, which the cell swallows by phagocytosis. These cells look a lot like choanoflagellates, the single-celled protists that DNA shows are animals' closest living relatives.
The body wall is just two thin cell layers with a jelly-like layer, the mesohyl, between them. Every cell is close to water, so oxygen, carbon dioxide and wastes simply diffuse in and out. Crawling amoebocytes carry food around, build the skeleton (hard spicules or flexible spongin fibers) and can turn into other cell types.
Most sponges make both eggs and sperm, usually acting as one sex first and then the other. Sperm leave in the outgoing water and get pulled into nearby sponges. The zygote grows into a swimming larva that settles and becomes a new adult.
Update: DNA studies now mostly agree that sponges form a single clade. Whether sponges or comb jellies were the first branch off the rest of the animal tree is still debated, and some recent genome studies favor comb jellies.

Key terms (12)

Porifera: The phylum of sponges: simple, sessile animals with no true tissues that filter food from water.
suspension feeder: An animal that eats by trapping small food particles floating in the water around it. Sponges, clams and barnacles all feed this way.
basal animal: A member of a lineage that branched off near the base of the animal family tree, before most other groups split apart.
true tissue: Specialized cells joined tightly together on a thin supporting layer so they work as one unit. Sponges don't have them; nearly all other animals do.
spongocoel: The central cavity inside a sponge where water collects after it comes in through the pores.
osculum: The large opening, usually at the top of a sponge, where water leaves the body.
choanocyte: A collar cell. Its flagellum drives water through the sponge, and its ring of projections traps food that it then engulfs.
choanoflagellate: A single-celled protist with a flagellum and a collar, very like a sponge's collar cell. DNA shows it's the closest living relative of animals.
mesohyl: The jelly-like layer between a sponge's two cell layers, where amoebocytes move around and eggs develop.
amoebocyte: A sponge cell that crawls through the mesohyl. It moves food, builds skeleton pieces and can become other kinds of sponge cells.
spicule: A tiny, hard, needle-like skeleton piece in many sponges, made of calcium carbonate or silica.
hermaphrodite: An individual that makes both eggs and sperm, either at the same time or one after the other.

Check yourself: 33.1 Sponges: animals without true tissues

4 questions on 33.1 Sponges: animals without true tissues. Pick an answer to see if you got it, and why.

Question 1 of 4

A biologist adds a chemical to the water around a sponge that stops all flagella from beating but doesn't kill any cells. Which result is most likely in the next hour?

Question 2 of 4

Researchers add fluorescent beads the size of bacteria to a tank holding a sponge. They then check which cells glow (invented data): Time after adding beads | Cells holding beads 10 minutes | Collar cells lining the inner chamber 2 hours | Collar cells and amoebocytes in the mesohyl 6 hours | Amoebocytes and cells near the outer surface Which interpretation best fits these results?

Question 3 of 4

Sponges have no lungs, gills, heart or blood vessels, yet some grow larger than a person. Which feature best explains how their cells get enough oxygen?

Question 4 of 4

Animals' closest living relatives are thought to be choanoflagellates, not fungi. Which evidence would most strongly support this hypothesis?

0 of 4 answered

33.2 Cnidarians: jellies, corals and their kin

pp. 671–673

On the AP exam? Not tested

Cnidarian body plans and groups aren't in the current course. They're great examples for tested ideas, though: the coral–algae mutualism (Topic 8.5), how ocean warming disrupts reefs (Topic 8.7) and how organisms respond to signals from their surroundings (Topic 8.1).

In the course: Topic 2.5 Membrane Transport, Topic 3.4 Photosynthesis, Topic 7.9 Phylogeny, Topic 8.1 Responses to the Environment, Topic 8.5 Community Ecology, Topic 8.7 Disruptions in Ecosystems (notes, videos and more questions)

Key points

Cnidarians include corals, sea anemones, hydras and jellies. They were one of the first lineages with true tissues, so they belong to the clade Eumetazoa, and their simple plan has lasted over half a billion years.
The body has just two tissue layers: an outer epidermis and an inner gastrodermis with a jelly-like mesoglea between. It's radially symmetrical, like a cup or bell, and wraps around a gut called the gastrovascular cavity, whose one opening is both mouth and anus.
There are two body forms. A polyp is a tube stuck down at its base with its tentacles facing up, like a sea anemone. A medusa is a free-floating bell with tentacles hanging down, like a jelly. Some species are only one form, and others switch between them during their life.
Cnidarians are predators. Their tentacles are packed with cnidocytes, cells that fire tiny capsules. The stinging kind, nematocysts, shoot out a thread that can pierce prey and inject venom. Enzymes in the gut cavity break prey down, and the lining cells take in the bits and finish digesting them.
There's no brain. A nerve net spreads through the body and picks up stimuli from every direction. Water held in the closed gut works as a fluid skeleton: muscle-like cells squeeze against it to stretch, shorten or bend the body.
Main groups: hydrozoans (many switch between polyp and medusa; freshwater hydras are only polyps), scyphozoans (mostly medusae, the classic jellies), cubozoans (box jellies: strong swimmers with complex eyes and very dangerous venom) and anthozoans (sea anemones and corals, which are only ever polyps).
Reef-building corals lay down calcium carbonate skeletons and get much of their food from photosynthetic algae living inside their cells. Water that stays too warm makes corals expel the algae and turn white, which is called bleaching. Repeated marine heat waves are now the biggest threat to reefs, on top of pollution and overfishing.

Key terms (12)

Cnidaria: The phylum of corals, sea anemones, hydras and jellies: two tissue layers, radial symmetry and stinging cells.
Eumetazoa: The clade of animals with true tissues. It includes cnidarians and bilaterians but not sponges.
diploblastic: Built from two embryonic tissue layers, ectoderm and endoderm, with no mesoderm. Cnidarians are diploblastic.
gastrovascular cavity: A gut with just one opening that is both mouth and anus. Food goes in and leftovers come out the same way.
polyp: The tube-shaped cnidarian form, attached at its base with its mouth and tentacles pointing up, like a sea anemone or a coral.
medusa: The bell-shaped, free-swimming cnidarian form, with its mouth and tentacles hanging down, like a jelly.
cnidocyte: A cell unique to cnidarians that shoots out a thread from a tiny capsule to sting, grab or snare prey.
nematocyst: A stinging capsule inside a cnidocyte. It shoots out a thread that can pierce prey and inject venom.
nerve net: A web of nerve cells spread through the body with no brain, so the animal can sense and respond in every direction.
hydrostatic skeleton: Support from fluid trapped in a closed space. Muscles push against the fluid to change the body's shape and move it.
anthozoan: A member of the cnidarian group that includes sea anemones and corals, which live only as polyps.
coral bleaching: When stressed corals, usually by heat, lose the algae living in their cells and turn white. If it lasts, the corals can starve and die.

Check yourself: 33.2 Cnidarians: jellies, corals and their kin

4 questions on 33.2 Cnidarians: jellies, corals and their kin. Pick an answer to see if you got it, and why.

Question 1 of 4

Researchers touch the tentacles of a sea anemone in several ways and count the share of touched cnidocytes that fire (invented data): Treatment | Cnidocytes that fired (%) Touched with a clean glass rod | 4 Touched with a rod coated in fish extract | 61 Fish extract added to the water, no touch | 3 Which conclusion do these data best support?

Question 2 of 4

Cells lining a sea anemone's gut cavity normally take in small bits of partly digested prey by endocytosis. Researchers treat an anemone with a drug that blocks only endocytosis, then feed it a small shrimp. Which result is most likely?

Question 3 of 4Calculator allowed

Question 4 of 4

A diver lightly touches one polyp in a coral colony. That polyp pulls in its tentacles, and within a few seconds a widening ring of neighboring polyps does the same, though the diver touched no others. Which explanation fits best?

0 of 4 answered

33.3 Lophotrochozoans: flatworms, molluscs, annelids and more

pp. 674–683

On the AP exam? Not tested

These phyla aren't in the current course. The tested ideas they illustrate are surface area and diffusion in thin bodies (Topic 2.2), parasites and hosts (Topic 8.5), invasive species (Topic 8.7) and groups defined by DNA (Topic 7.9).

In the course: Topic 2.2 Cell Size, Topic 3.5 Cellular Respiration, Topic 5.2 Meiosis and Genetic Diversity, Topic 7.2 Natural Selection, Topic 7.9 Phylogeny, Topic 8.5 Community Ecology, Topic 8.7 Disruptions in Ecosystems (notes, videos and more questions)

Key points

Most animals are bilaterians: they have left and right sides, three embryonic tissue layers, and usually a gut running from mouth to anus. DNA sorts them into three big clades: lophotrochozoans, ecdysozoans and deuterostomes.
Lophotrochozoans were grouped by DNA, not by one body part they all share. Some have a lophophore (a ring of ciliated feeding tentacles), some have a trochophore larva, and many have neither. With about 18 phyla, they vary in body plan more than any other bilaterian clade.
Flatworms are thin, with no body cavity, and most have a gut with one opening. Being flat keeps every cell near the outside or the gut, so oxygen and ammonia diffuse in and out without lungs or blood vessels. Planarians live freely; flukes and tapeworms are parasites, and tapeworms have no gut at all, absorbing digested food from their host across their skin.
Rotifers are tinier than many protists but are multicellular, with a crown of cilia, grinding jaws and a complete gut inside a pseudocoelom. Many can produce daughters from unfertilized eggs, which is called parthenogenesis.
Ectoprocts (bryozoans) and brachiopods stay in one place and filter food with a lophophore. Ectoprocts grow as colonies, often with a hard outer covering; brachiopods have a two-part shell.
Molluscs share one plan: a muscular foot, a visceral mass holding the organs, and a mantle that can make a shell. Most scrape food with a toothed radula. Chitons, snails, clams and squids and octopuses reshape that plan in different ways, and only cephalopods (squids, octopuses and their kin) have a closed circulatory system and a large brain. Molluscs have more recorded extinctions than any other animal group, mostly from habitat loss, pollution and introduced species.
Annelids are segmented worms with a true coelom and closed circulation. Update: this edition splits them into polychaetes and oligochaetes, but DNA now shows that earthworms and leeches evolved from within the bristly, mostly marine worms, so 'polychaetes' is not a clade.

Key terms (15)

bilaterian: An animal with bilateral symmetry and three embryonic tissue layers. Most animals, from worms to insects to people, are bilaterians.
Lophotrochozoa: A large clade of bilaterians grouped by DNA, including flatworms, rotifers, molluscs and annelids.
lophophore: A ring or horseshoe of tentacles covered in cilia that some animals use to catch food floating in the water.
trochophore larva: A tiny, swimming larva with bands of cilia, found in many marine molluscs and annelids.
acoelomate: An animal with no body cavity between its gut and its outer body wall, like a flatworm.
Platyhelminthes: The phylum of flatworms: thin bodies, no body cavity and usually a gut with one opening. It includes planarians, flukes and tapeworms.
tapeworm: A parasitic flatworm that lives in a host's intestine. It has no mouth or gut and absorbs nutrients across its body surface.
pseudocoelom: A fluid-filled body cavity that is only partly lined by mesoderm, found in rotifers and nematodes.
alimentary canal: A complete gut: a tube with a mouth at one end and an anus at the other, so food moves one way.
parthenogenesis: Reproduction in which an egg develops into a new individual without being fertilized.
mantle: In molluscs, the fold of tissue that covers the organs and makes the shell, if there is one.
radula: A tongue-like ribbon of tiny teeth that many molluscs use to scrape or cut food.
visceral mass: The part of a mollusc's body that holds most of its internal organs.
closed circulatory system: A system in which blood stays inside vessels the whole time. Annelids, squids, octopuses and vertebrates have one.
Annelida: The phylum of segmented worms, including earthworms, leeches and many marine bristle worms.

Check yourself: 33.3 Lophotrochozoans: flatworms, molluscs, annelids and more

4 questions on 33.3 Lophotrochozoans: flatworms, molluscs, annelids and more. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

A student models a flatworm as a flat sheet 10 mm long, 4 mm wide and 0.2 mm thick (volume 8 mm³, surface area 85.6 mm²). She compares it with a round worm shape of the same length and volume, whose surface area is about 33.3 mm². Which statement is correct?

Question 2 of 4

A lab culture is started from one female rotifer of a species that reproduces only by parthenogenesis. Her eggs form by mitosis-like divisions and are never fertilized. After 30 generations, which outcome is most likely?

Question 3 of 4

At the same temperature, a squid uses oxygen many times faster per kilogram of body mass than a clam does. Which feature of the squid best matches this high demand?

Question 4 of 4

0 of 4 answered

33.4 Ecdysozoans: nematodes and arthropods

pp. 683–692

On the AP exam? Not tested

You won't be asked about nematode or arthropod anatomy, but these animals are common exam examples: pollinators and parasites in Topic 8.5, behavior in Topic 8.1, gas exchange and body size in Topic 2.2, and transcription factors shaping body plans in Topic 6.6.

In the course: Topic 2.2 Cell Size, Topic 6.6 Gene Expression and Cell Specialization, Topic 7.9 Phylogeny, Topic 8.1 Responses to the Environment, Topic 8.5 Community Ecology (notes, videos and more questions)

Key points

Ecdysozoans are grouped by DNA and named for ecdysis, or molting: as they grow, they shed a tough outer layer called the cuticle. The clade has about eight phyla, and it holds more named species than every plant, fungus, protist and other animal group combined.
Nematodes (roundworms) are unsegmented, tube-shaped worms coated in a cuticle. They have a complete gut but no circulatory system, so fluid in the pseudocoelom carries nutrients. They live almost everywhere, recycle nutrients in soil, and include parasites of crops, animals and people. One soil species, C. elegans, is a major lab model.
Arthropods (insects, spiders, crustaceans, centipedes and kin) are the most species-rich phylum. Their success is linked to a segmented body, jointed appendages and an exoskeleton of chitin and protein. Over time, segments fused into body regions, and appendages became tools for walking, feeding, sensing, mating or defense. Shifts in how old Hox genes work, not brand-new genes, are thought to drive much of this.
The exoskeleton protects the body, anchors muscles and slows water loss on land. The catch: to grow, an arthropod must molt, which costs energy and leaves it soft and easy to attack until the new cuticle hardens.
Arthropod circulation is open: a heart pumps a fluid called hemolymph into spaces around the organs. Water-dwellers usually breathe with gills; land-dwellers use internal surfaces, like the branching tracheal tubes of insects or the stacked book lungs of many spiders.
Living arthropods form three main lineages: chelicerates (spiders, scorpions, ticks, mites and horseshoe crabs, with fang-like chelicerae and no antennae), myriapods (centipedes and millipedes) and a big clade of crustaceans plus insects. Update: this edition treats crustaceans and insects as separate lineages, but DNA shows insects evolved from within the crustaceans, and together they form the clade Pancrustacea.
Insects have a head, thorax and abdomen, three pairs of legs and, often, wings. Flight and spreading alongside flowering plants helped make them the most diverse animals. Many go through metamorphosis, either complete (larva, pupa, then a very different adult) or incomplete (nymphs that look like small adults). Insects pollinate crops, eat plants, feed other animals and carry diseases.

Key terms (15)

Ecdysozoa: A huge clade of animals, grouped by DNA, that molt as they grow. Nematodes and arthropods are the biggest phyla in it.
ecdysis: Molting: shedding an old outer covering and making a new, larger one so the animal can grow.
cuticle: A tough, nonliving outer coat made by the skin cells. Nematodes and arthropods shed it as they grow.
Nematoda: The phylum of roundworms: unsegmented worms with a cuticle, a complete gut and no circulatory system.
Arthropoda: The phylum of animals with a segmented body, jointed legs and an exoskeleton, such as insects, spiders and crabs.
exoskeleton: A hard covering on the outside of the body that protects it and gives muscles something to pull on.
chitin: A tough, nitrogen-containing polysaccharide in arthropod exoskeletons and fungal cell walls.
open circulatory system: A system in which the heart pumps fluid out of vessels into body spaces, where it bathes the organs directly.
hemolymph: The fluid in an open circulatory system, doing the jobs of both blood and the fluid around cells.
tracheal system: A network of branching air tubes in insects that carries oxygen from openings in the body wall right to the cells.
Hox genes: Genes for transcription factors that tell each region of an embryo what body parts to build along the head-to-tail axis.
chelicerae: Pincer- or fang-like feeding parts in front of the mouth of spiders, scorpions, ticks and their relatives.
Pancrustacea: The clade made of crustaceans and insects, since DNA shows insects branched off from within the crustaceans.
complete metamorphosis: Development with a larva, a pupa and an adult that looks very different, as in butterflies and beetles.
incomplete metamorphosis: Development in which young nymphs look like small adults and change gradually with each molt, as in grasshoppers.

Check yourself: 33.4 Ecdysozoans: nematodes and arthropods

4 questions on 33.4 Ecdysozoans: nematodes and arthropods. Pick an answer to see if you got it, and why.

Question 1 of 4

Researchers study crayfish at different points in their molting cycle (invented data): Condition | Eaten by a predatory fish in 24 h, tanks with no shelter (%) | Time spent in shelters, tanks with shelters (%) More than 10 days after a molt | 8 | 30 0–2 days after a molt | 46 | 85 Which explanation fits both results best?

Question 2 of 4

Beetle larvae of one species are raised in air with different oxygen levels, and the adults are weighed (invented data): Oxygen in air (%) | Mean adult mass (mg) 12 | 82 21 | 100 35 | 109 Which hypothesis do these results support?

Question 3 of 4

In a beetle, one Hox gene is normally switched on only in the abdomen, which grows no legs. Researchers use RNA interference to destroy that gene's mRNA in embryos, and the larvae that hatch have small legs on their abdominal segments (invented results). Which conclusion fits best?

Question 4 of 4

Students test whether insect visits are needed for blueberry flowers to make fruit. On each of 20 bushes, they cover one flower cluster with a fine mesh bag that keeps insects out and leave another cluster on the same bush uncovered. Why is it better to have both treatments on the same bushes?

0 of 4 answered

33.5 Echinoderms and the deuterostome clade

pp. 692–694

On the AP exam? Background

Echinoderm anatomy isn't tested, but this section is good practice for Topic 7.9: groups are defined by shared ancestry shown in DNA, not by looks, which is why a sea urchin is a closer relative of yours than a crab is. Sea star die-offs also make a strong keystone-species example for Topic 8.6.

In the course: Topic 7.6 Evidence of Evolution, Topic 7.9 Phylogeny, Topic 8.6 Biodiversity, Topic 8.7 Disruptions in Ecosystems (notes, videos and more questions)

Key points

Deuterostomes include echinoderms (sea stars, sea urchins and their relatives), hemichordates (acorn worms) and chordates, the phylum that includes us. What unites them is DNA evidence of shared ancestry, not body shape.
Many deuterostomes share a way of developing: radial cleavage in the early embryo, and the first opening (the blastopore) becomes the anus. But some animals outside the clade develop this way too, so development alone can't define the group.
Echinoderms all live in the sea. Under a thin skin they have an endoskeleton of hard calcium carbonate plates, often studded with bumps and spines.
Only echinoderms have a water vascular system, a web of fluid-filled canals that ends in hundreds of tube feet. Muscles push water into the tube feet to extend them, and the feet grip using sticky chemicals, not suction, and let go by releasing other chemicals.
Adults usually have five-part radial symmetry, but their larvae are bilateral, a clue that echinoderms come from bilateral ancestors. Even adults aren't perfectly radial.
Five living groups: sea stars (arms around a central disk, plus the tiny armless sea daisies), brittle stars (thin, snaking arms), sea urchins and sand dollars (no arms, with tube feet in five bands), sea lilies and feather stars (filter feeders with an ancient body plan) and sea cucumbers (long, soft bodies with a much-reduced skeleton).
Echinoderms and chordates are sister lineages that split more than half a billion years ago, so one didn't give rise to the other. Echinoderms can matter a lot in ecosystems: since 2013, a wasting disease has killed billions of sea stars on North America's Pacific coast, and the loss of urchin-eating sunflower sea stars helped urchins multiply and strip many kelp forests.

Key terms (12)

deuterostome: A member of the bilaterian clade that includes echinoderms, hemichordates and chordates, grouped mainly by DNA.
Echinodermata: The phylum of sea stars, brittle stars, sea urchins, sea lilies and sea cucumbers: marine animals with tube feet and a skeleton of hard plates.
blastopore: The first opening that forms in an early embryo. In most deuterostomes it becomes the anus.
radial cleavage: A pattern of early cell division in which the new cells stack neatly on top of each other, typical of many deuterostomes.
endoskeleton: A hard skeleton inside the body, covered by living tissue, that grows along with the animal.
water vascular system: A network of water-filled canals found only in echinoderms. It runs the tube feet used for moving and feeding.
tube foot: One of hundreds of small, water-filled extensions of an echinoderm's water vascular system. Tube feet grip, move and handle food.
madreporite: A small sieve-like plate where water passes between an echinoderm's water vascular system and the sea.
pentaradial symmetry: Body parts arranged in five sections around a center, like the arms of a sea star.
hemichordate: A small group of worm-shaped marine deuterostomes, such as acorn worms, that share some features with chordates.
Chordata: The phylum that includes vertebrates like us, plus lancelets and tunicates, all deuterostomes.
sea star wasting disease: A disease that turns sea stars into mush. Since 2013 it has killed billions of them along the Pacific coast of North America.

Check yourself: 33.5 Echinoderms and the deuterostome clade

4 questions on 33.5 Echinoderms and the deuterostome clade. Pick an answer to see if you got it, and why.

Question 1 of 4

A family tree built from DNA shows that crabs branched off first, and sea urchins and humans then share a more recent common ancestor with each other. Which statement does this tree support?

Question 2 of 4

Adult sea stars have five-part radial symmetry, but their swimming larvae are bilaterally symmetrical, and DNA places sea stars inside the bilaterian clade. Which conclusion is best supported?

Question 3 of 4

Question 4 of 4

Arrow worms are small marine predators whose embryos develop much like deuterostome embryos. Yet sequences of hundreds of genes place them among the protostomes. How should a biologist classify them?

0 of 4 answered