Chapter 33 Invertebrates
Overview: Life Without a Backbone
• Invertebrates—animals without a backbone—account for 95% of
known animal species and all but one of the roughly 35 animal phyla
that have been described.
• More than a million extant species of animals are known, and at
least as many more will probably be identified by future
• Invertebrates inhabit nearly all environments on Earth, from the
scalding water of deep-sea hydrothermal vents to the rocky, frozen
ground of Antarctica.
Concept 33.1 Sponges are sessile and have a porous body and
• Sponges (phylum Porifera) are so sedentary that they were mistaken
for plants by the early Greeks.
• Living in freshwater and marine environments, sponges are
• The body of a simple sponge resembles a sac perforated with holes.
• Water is drawn through the pores into a central cavity, the
spongocoel, and flows out through a larger opening, the
• More complex sponges have folded body walls, and many
contain branched water canals and several oscula.
• Sponges range in height from about a few mm to 2 m and most are
• About 100 species live in fresh water.
• Unlike eumetazoa, sponges lack true issues, groups of similar cells
that form a functional unit. • The germ layers of sponges are loose federations of cells, which are
not really tissues because the cells are relatively unspecialized.
• The sponge body does contain different cell types.
• Sponges collect food particles from water passing through food-
• Flagellated choanocytes, or collar cells, lining the spongocoel
(internal water chambers) create a flow of water through the
sponge with their flagella and trap food with their collars.
• Based on both molecular evidence and the morphology of their
choanocytes, sponges evolved from a colonial choanoflagellate
• The body of a sponge consists of two cell layers separated by a
gelatinous region, the mesohyl.
• Wandering though the mesohyl are amoebocytes.
• They take up food from water and from choanocytes, digest it,
and carry nutrients to other cells.
• They also secrete tough skeletal fibers within the mesohyl.
• In some groups of sponges, these fibers are sharp
spicules of calcium carbonate or silica.
• Other sponges produce more flexible fibers from a
collagen protein called spongin.
• ? We use these pliant, honeycombed skeletons as
• Most sponges are sequential hermaphrodites, with each individual
producing both sperm and eggs in sequence.
• Gametes arise from choanocytes or amoebocytes.
• The eggs are retained, but sperm are carried out the osculum
by the water current.
• Sperm are drawn into neighboring individuals and fertilize eggs
in the mesohyl. • The zygotes develop into flagellated, swimming larvae that
disperse from the parent.
• When a larva finds a suitable substratum, it develops into a
• Sponges produce a variety of antibiotics and other defensive
• Researchers are now isolating these compounds, which may
be useful in fighting human disease.
Concept 33.2 Cnidarians have radial symmetry, a gastrovascular
cavity, and cnidocytes
• All animals except sponges belong to the Eumetazoa, the animals
with true tissues.
• The cnidarians (hydras, jellies, sea anemones, and coral animals)
have a relatively simple body construction.
• They are a diverse group with more than 10,000 living species,
most of which are marine.
• They exhibit a relatively simple, diploblastic body plan that
arose 570 million years ago.
• The basic cnidarian body plan is a sac with a central digestive
compartment, the gastrovascular cavity.
• A single opening to this cavity functions as both mouth and
• This basic body plan has two variations: the sessile polyp and the
• The cylindrical polyps, such as hydras and sea anemones, adhere to
the substratum by the aboral end and extend their tentacles, waiting
• Medusas (also called jellies) are flattened, mouth-down versions of
polyps that move by drifting passively and by contracting their bell-
shaped bodies. • The tentacles of a jelly dangle from the oral surface.
• Some cnidarians exist only as polyps.
• Others exist only as medusas.
• Still others pass sequentially through both a medusa stage and
a polyp stage in their life cycle.
• Cnidarians are carnivores that use tentacles arranged in a ring
around the mouth to capture prey and push the food into the
gastrovascular chamber for digestion.
• Batteries of cnidocytes on the tentacles defend the animal or
• Organelles called cnidae evert a thread that can inject
poison into the prey, or stick to or entangle the target.
• Cnidae called nematocysts are stinging capsules.
• Muscles and nerves exist in their simplest forms in cnidarians.
• Cells of the epidermis and gastrodermis have bundles of
microfilaments arranged into contractile fibers.
• True muscle tissue appears first in triploblastic animals.
• When the animal closes its mouth, the gastrovascular cavity
acts as a hydrostatic skeleton against which the contractile cells
• Movements are controlled by a noncentralized nerve net associated
with simple sensory receptors that are distributed radially around the
• The phylum Cnidaria is divided into four major classes: Hydrozoa,
Scyphozoa, Cubozoa, and Anthozoa.
• The four cnidarian classes show variations on the same body theme
of polyp and medusa.
• Most hydrozoans alternate polyp and medusa forms, as in the life
cycle of Obelia. • The polyp stage, often a colony of interconnected polyps, is
more conspicuous than the medusa.
• Hydras, among the few freshwater cnidarians, are unusual members
of the class Hydrozoa in that they exist only in the polyp form.
• When environmental conditions are favorable, a hydra
reproduces asexually by budding, the formation of outgrowths
that pinch off from the parent to live independently.
• When environmental conditions deteriorate, hydras form
resistant zygotes that remain dormant until conditions improve.
• The medusa generally prevails in the life cycle of class Scyphozoa.
• The medusae of most species live among the plankton as
• Most coastal scyphozoans go through small polyp stages during their
• Jellies that live in the open ocean generally lack the sessile
• Cubozoans have a box-shaped medusa stage.
• They can be distinguished from scyphozoans in other
significant ways, such as having complex eyes in the fringe of
• Cubozoans, which generally live in tropical oceans, are often
equipped with highly toxic cnidocytes.
• Sea anemones and corals belong to the class Anthozoa.
• They occur only as polyps.
• Coral animals live as solitary or colonial forms and secrete a
hard external skeleton of calcium carbonate.
• Each polyp generation builds on the skeletal remains of earlier
generations to form skeletons that we call coral.
• In tropical seas, coral reefs provide habitat for a great diversity of
invertebrates and fishes. • Coral reefs in many parts of the world are currently being
destroyed by human activity.
• Pollution, overfishing, and global warming are contributing to
Concept 33.3 Most animals have bilateral symmetry
• The vast majority of animal species belong to the clade Bilateria,
which consists of animals with bilateral symmetry and triploblastic
• Most bilaterians are also coelomates.
• The most recent common ancestor of living bilaterians probably lived
in the later Proterozoic.
• During the Cambrian explosion, most major groups of bilaterians
Phylum Platyhelminthes: Flatworms are acoelomates with
• Flatworms live in marine, freshwater, and damp terrestrial habitats.
• They also include many parasitic species, such as the flukes
• Flatworms have thin bodies, ranging in size from nearly microscopic
to tapeworms more than 20 m long.
• Flatworms and other bilaterians are triploblastic, with a middle
embryonic tissue layer, a mesoderm, which contributes to more
complex organs and organ systems and to true muscle tissue.
• While flatworms are structurally more complex than cnidarians, they
are simpler than other bilaterians.
• Like cnidarians, flatworms have a gastrovascular cavity with
only one opening (and tapeworms lack a digestive system
entirely and absorb nutrients across their body surface).
• Unlike other bilaterians, flatworms lack a coelom. • The flat shape of a flatworm places all cells close to the surrounding
water, enabling gas exchange and the elimination of nitrogenous
wastes (ammonia) by diffusion across the body surface.
• Flatworms have no specialized organs for gas exchange and
circulation, and their relatively simple excretory apparatus functions
mainly to maintain osmotic balance.
• This apparatus consists of ciliated cells called flame bulbs that
waft fluid through branched ducts that open to the outside.
• Flatworms are divided into four classes: Turbellaria, Monogenia,
Trematoda, and Cestoidea.
• Turbellarians are nearly all free-living (nonparasitic) and most are
• Planarians, members of the genus Dugesia, are carnivores or
scavengers in unpolluted ponds and streams.
• Planarians move using cilia on the ventral epidermis, gliding along a
film of mucus they secrete.
• Some turbellarians use muscles for undulatory swimming.
• A planarian has a head with a pair of eyespots to detect light, and
lateral flaps that function mainly for smell.
• The planarian nervous system is more complex and centralized
than the nerve net of cnidarians.
• Planarians can learn to modify their responses to stimuli.
• Planarians reproduce asexually through regeneration.
• The parent constricts in the middle, and each half
regenerates the missing end.
• Planarians can also reproduce sexually.
• These hermaphrodites cross-fertilize.
• The monogeneans (class Monogenea) and the trematodes
(class Trematoda) live as parasites in or on other animals.
• Many have suckers for attachment to their host. • A tough covering protects the parasites.
• Reproductive organs nearly fill the interior of these
• Trematodes parasitize a wide range of hosts, and most species
have complex life cycles with alternation of sexual and asexual
• Many require an intermediate host in which the larvae
develop before infecting the final hosts (usually a
vertebrate) where the adult worm lives.
• The blood fluke Schistosoma infects 200 million people,
leading to body pains and dysentery.
• The intermediate host for Schistosoma is a snail.
• Living within different hosts puts demands on trematodes that
free-living animals do not face.
• A blood fluke must evade the immune systems of two
very different hosts.
• By mimicking their host’s surface proteins, blood flukes
create a partial immunological camouflage.
• They also release molecules that manipulate the host’s
• These defenses are so effective that individual flukes can
survive in a human host for more than 40 years.
• Most monogeneans are external parasites of fishes.
• Their life cycles are simple, with a ciliated, free-living larva that
starts an infection on a host.
• While traditionally aligned with trematodes, some structural and
chemical evidence suggests that they are more closely related
• Tapeworms (class Cestoidea) are also parasitic.
• The adults live mostly in vertebrates, including humans. • Suckers and hooks on the head, or scolex, anchor the worm in the
digestive tract of the host.
• Tapeworms lack a gastrovascular cavity and absorb food
particles from their hosts.
• A long series of proglottids, sacs of sex organs, lie posterior to the
• Mature proglottids, loaded with thousands of eggs, are released
from the posterior end of the tapeworm and leave with the
• In one type of cycle, tapeworm eggs in contaminated food or
water are ingested by intermediary hosts, such as pigs or cattle.
• The eggs develop into larvae that encyst in the muscles of their
• Humans acquire the larvae by eating undercooked meat
contaminated with cysts.
• The larvae develop into mature adults within the human.
Phylum Rotifera: Rotifers are pseudocoelomates with jaws, crowns of
cilia, and complete digestive tracts.
• Rotifers are tiny animals (5 µm to 2 mm), most of which live in
• Some live in the sea or in damp soil.
• Rotifers are smaller than many protists but are truly multicellular, with
specialized organ systems.
• Rotifers have an alimentary canal, a digestive tract with a separate
mouth and anus.
• Internal organs lie in the pseudocoelom, a body cavity that is not
completely lined with mesoderm.
• The fluid in the pseudocoelom serves as a hydrostatic skeleton. • Through the movements of nutrients and wastes dissolved in
the coelomic fluid, the pseudocoelom also functions as a
• The word rotifer, “wheel-bearer,” refers to the crown of cilia that
draws a vortex of water into the mouth.
• Food particles drawn in by the cilia are captured by the jaws
(trophi) in the pharynx and ground up.
• Some rotifers exist only as females that produce more females from
unfertilized eggs, a type of parthenogenesis.
• Other species produce two types of eggs that develop by
• One type forms females, and the other forms degenerate males
that survive just long enough to fertilize eggs.
• The zygote forms a resistant stage that can withstand
environmental extremes until conditions improve.
• The zygote then begins a new female generation that
reproduces by parthenogenesis until conditions become
• It is puzzling that so many rotifers survive without males.
• The vast majority of animals and plants reproduce sexually at
least some of the time, and sexual reproduction has certain
advantages over asexual reproduction.
• For example, species that reproduce asexually tend to
accumulate harmful mutations in their genomes faster than
sexually reproducing species.
• As a result, asexual species experience higher rates of
extinction and lower rates of speciation.
• A class of asexual rotifers called Bdelloidea consists of 360 species
that all reproduce by parthenogenesis without males.
• Thirty-five-million-year-old bdelloid rotifers have been found
preserved in amber. • The morphology of these fossils resembles the female form.
• DNA comparisons of bdelloids with their closest sexually
reproducing rotifer relatives suggest that bdelloids have been
asexual for far more than 35 million years.
• Bdelloid rotifers raise interesting questions about the evolution of sex.
The lophophorate phyla: ectoprocts, phoronids, and brachiopods are
coelomates with ciliated tentacles around their mouths.
• Bilaterians in three phyla—Ectoprocta, Phoronida, and
Brachiopoda—are traditionally called lophophorate animals because
they all have a lophophore.
• The lophophore is a horseshoe-shaped or circular fold of the
body wall bearing ciliated tentacles that surround and draw
water toward the mouth.
• The tentacles trap suspended food particles.
• In addition to the lophophore, these three phyla share a U-shaped
digestive tract and the absence of a head.
• These may be adaptations to a sessile existence.
• In contrast to flatworms, which lack a body cavity, and rotifers, which
have a pseudocoelom, lophophorates have true coeloms completely
lined with mesoderm.
• Ectoprocts are colonial animals that superficially resemble plants.
• In most species, the colony is encased in a hard exoskeleton.
• The lophophores extend through pores in the exoskeleton.
• Most ectoprocts are marine, where they are widespread and
numerous sessile animals, with several species that can be important
• Ectoprocts also live in lakes and rivers.
• Phoronids are tube-dwelling marine worms ranging from 1 mm to 50
cm in length.
• Some live buried in the sand within chitinous tubes. • They extend the lophophore from the tube when feeding and
pull it back in when threatened.
• Brachiopods, or lampshells, superficially resemble clams and other
• However, the two halves of the brachiopod are dorsal and
ventral to the animal, rather than lateral as in clams.
• All brachiopods are marine.
• Most live attached to the substratum by a stalk, opening their
shell slightly to allow water to flow over the lophophore.
• The living brachiopods are remnants of a richer past.
• Thirty thousand species of brachiopod fossils have been
described from the Paleozoic and Mesozoic eras.
Phylum Nemertea: Proboscis worms are named for their prey-
• The members of the Phylum Nemertea, proboscis worms or ribbon
worms, have bodies much like those of flatworms.
• However, they have a small fluid-filled sac that may be a
reduced version of a true coelom.
• The sac and fluid hydraulics operate an extensible proboscis,
which the worm uses to capture prey.
• Nemerteans range in length from less than 1 mm to several meters.
• Nearly all nemerteans are marine, but a few species inhabit fresh
water or damp soil.
• Some are active swimmers, and others burrow into the sand.
• Nemerteans and flatworms have similar excretory, sensory, and
• However, nemerteans have an alimentary canal and a closed
circulatory system in which the blood is contained in vessels.
• Nemerteans have no heart, and the blood is propelled by
muscles squeezing the vessels. Concept 33.4 Molluscs have a muscular foot, a visceral mass, and a
• The phylum Mollusca includes many diverse forms, including snails
and slugs, oysters and clams, and octopuses and squids.
• Most molluscs are marine, though some inhabit fresh water, and
some snails and slugs live on land.
• Molluscs are soft-bodied animals, but most are protected by a hard
shell of calcium carbonate.
• Slugs, squids, and octopuses have reduced or lost their shells
completely during their evolution.
• Despite their apparent differences, all molluscs have a similar body
plan with a muscular foot (typically for locomotion), a visceral mass
with most of the internal organs, and a mantle.
• The mantle, which secretes the shell, drapes over the visceral
mass and creates a water-filled chamber, the mantle cavity,
with gills, anus, and excretory pores.
• Many molluscs feed by using a straplike rasping organ, a
radula, to scrape up food.
• Most molluscs have separate sexes, with gonads located in the
• However, many snails are hermaphrodites.
• The life cycle of many marine molluscs includes a ciliated larva, the
• This larva is also found in marine annelids (segmented worms)
and some other lophotrochozoans.
• The basic molluscan body plan has evolved in various ways in the
eight classes of the phylum.
• The four most prominent are the Polyplacophora (chitons),
Gastropoda (snails and slugs), Bivalvia (clams, oysters, and
other bivalves), and Cephalopoda (squids, octopuses,
cuttlefish, and chambered nautiluses). • Chitons are marine animals with oval shapes and shells divided into
eight dorsal plates.
• The chiton body is unsegmented.
• Chitons use their muscular foot to grip the rocky substrate tightly and
to creep slowly over the rock surface.
• Chitons are grazers that use their radulas to scrape and ingest algae.
• Almost three-quarters of all living species of molluscs are gastropods.
• Most gastropods are marine, but there are also many
• Garden snails and slugs have adapted to land.
• During embryonic development, gastropods undergo torsion in which
the visceral mass is rotated up to 180 degrees, so the anus and
mantle cavity are above the head in adults.
• After torsion, some of the organs that were bilateral are
reduced or lost on one side of the body.
• Most gastropods are protected by single, spiral shells into which the
animals can retreat if threatened.
• Torsion and formation of the coiled shell are independent
• While gastropod shells are typically conical, those of abalones and
limpets are somewhat flattened.
• Many gastropods have distinct heads with eyes at the tips of
• They move by a rippling motion of their foot or by means of cilia.
• Most gastropods use their radula to graze on algae or plant material.
• Some species are predators.
• In these species, the radula is modified to bore holes in the
shells of other organisms or to tear apart tough animal tissues. • In the tropical marine cone snails, teeth on the radula form
separate poison darts, which penetrate and stun their prey,
• In place of the gills found in most aquatic gastropods, the lining of the
mantle cavity of terrestrial snails functions as a lung.
• The class Bivalvia includes clams, oysters, mussels, and scallops.
• Bivalves have shells divided into two halves.
• The two parts are hinged at the mid-dorsal line, and powerful
adductor muscles close the shell tightly to protect the animal.
• Bivalves have no distinct head, and the radula has been lost.
• Some bivalves have eyes and sensory tentacles along the
outer edge of the mantle.
• The mantle cavity of a bivalve contains gills that are used for feeding
and gas exchange.
• Most bivalves are suspension feeders, trapping fine particles in
mucus that coats the gills.
• Cilia convey the particles to the mouth.
• Water flows into the mantle cavity via the incurrent siphon,
passes over the gills, and exits via the excurrent siphon.
• Most bivalves live rather sedentary lives, a characteristic suited to
• Sessile mussels secrete strong threads that tether them to
rocks, docks, boats, and the shells of other animals.
• Clams can pull themselves into the sand or mud, using the
muscular foot as an anchor.
• Scallops can swim in short bursts to avoid predators by flapping
their shells and jetting water out their mantle cavity.
• Cephalopods are active predators that use rapid movements to dart
toward their prey, which they capture with several long tentacles. • Squids and octopuses use beak-like jaws to bite their prey and
then inject poison to immobilize the victim.
• A mantle covers the visceral mass, but the shell is reduced and
internal in squids, missing in many octopuses, and exists externally
only in chambered nautiluses.
• Fast movements by a squid occur when it contracts its mantle cavity
and fires a stream of water through the excurrent siphon.
• By pointing the siphon in different directions, the squid can
rapidly move in different directions.
• The foot of a cephalopod has been modified into the muscular siphon
and parts of the tentacles and head.
• Cephalopods are the only molluscs with a closed circulatory system.
• They also have well-developed sense organs and a complex
• The ancestors of octopuses and squid were probabl