1. Chapter 33
Invertebrates

2. Overview: Life Without a Backbone Invertebrates
Are animals that lack a backbone
Account for 95% of known animal species
Figure 33.1

3. A review of animal phylogeny
Ancestral colonial
choanoflagellate
Eumetazoa
Bilateria
Deuterostomia
Porifera
Cnidaria
Other bilaterians (including
Nematoda, Arthropoda,
Mollusca, and Annelida)
Echinodermata
Chordata
Figure 33.2

4. Exploring invertebrate diversity
PORIFERA (5,500 species)
A sponge
CNIDARIA (10,000 species)
A jelly
PLACOZOA (1 species)
KINORHYNCHA (150 species)
0.5 mm
A placozoan (LM)
A kinorhynch (LM)
250 µm
PLATYHELMINTHES (20,000 species)
ROTIFERA (1,800 species)
A marine flatworm
A rotifer (LM)
ECTOPROCTA (4,500 species)
PHORONIDA (20 species)
Ectoprocts
Phoronids
Figure 33.3

5. A ctenophore, or comb jelly
Exploring invertebrate diversity
BRACHIOPODA (335 species)
NEMERTEA (900 species)
A brachiopod
A ribbon worm
ACANTHOCEPHALA (1,100 species)
CTENOPHORA (100 species)
An acanthocephalan
A ctenophore, or comb jelly
MOLLUSCA (93,000 species)
ANNELIDA (16,500 species)
An octopus
A marine annelid
LORICIFERA (10 species)
PRIAPULA (16 species)
5 mm
50 µm
A loriciferan (LM)
A priapulan
Figure 33.3

6. A cycliophoran (colorized SEM) Tardigrades (colorized SEM)
Exploring invertebrate diversity
NEMATODA (25,000 species)
ARTHROPODA (1,000,000 + species)
A roundworm
A scorpion (an arachnid)
CYCLIOPHORA (1 species)
TARDIGRADA (800 species)
100 µm
A cycliophoran (colorized SEM)
Tardigrades (colorized SEM)
ONYCHOPHORA (110 species)
HEMICHORDATA (85 species)
An onychophoran
An acorn worm
ECHINODERMATA (7,000 species)
CHORDATA (52,000 species)
A sea urchin
A tunicate
Figure 33.3

7. Sponges are sedentary
Have a porous body and choanocytes
Sponges, phylum Calcarea & Silicea
Mistaken for plants
Among simplest animals
Live in both fresh and marine waters
Lack true tissues and organs

8. Sponges are suspension feeders
Capturing food particles suspended in the water that passes through their body
Azure vase sponge (Callyspongia
plicifera)
Osculum
Spicules
Water
flow
Flagellum
Collar
Food particles
in mucus
Choanocyte
Phagocytosis of
food particles
Amoebocyte
Choanocytes. The spongocoel
is lined with feeding cells called
choanocytes. By beating flagella,
the choanocytes create a current that
draws water in through the porocytes.
Spongocoel. Water
passing through porocytes
enters a cavity called the
spongocoel.
Porocytes. Water enters
the epidermis through
channels formed by
porocytes, doughnut-shaped
cells that span the body wall.
Epidermis. The outer
layer consists of tightly
packed epidermal cells.
Mesohyl. The wall of this
simple sponge consists of
two layers of cells separated
by a gelatinous matrix, the
mesohyl (“middle matter”).
The movement of the choanocyte
flagella also draws water through its
collar of fingerlike projections. Food
particles are trapped in the mucus
coating the projections, engulfed by
phagocytosis, and either digested or
transferred to amoebocytes.
Amoebocyte. Amoebocytes
transport nutrients to other cells of
the sponge body and also produce
materials for skeletal fibers (spicules). 5 6 7 4 3 2 1
Figure 33.4

9. Choanocytes, flagellated collar cells
Generate a water current through the sponge and ingest suspended food
Most sponges are hermaphrodites
Meaning that each individual functions as both male and female

10. All animals except sponges
Concept 33.2: Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes
All animals except sponges
Belong to the clade Eumetazoa, the animals with true tissues
Phylum Cnidaria
Is one of the oldest groups in this clade

11. Cnidarians
Have diversified into a wide range of both sessile and floating forms including jellies, corals, and hydras
But still exhibit a relatively simple diploblastic, radial body plan

12. The basic body plan of a cnidarian
Is a sac with a central digestive compartment, the gastrovascular cavity
A single opening
Functions as both mouth and anus

13. There are two variations on this body plan
The sessile polyp and the floating medusa
Mouth/anus
Tentacle
Gastrovascular
cavity
Gastrodermis
Mesoglea
Epidermis
Body
stalk
Medusa
Polyp
Figure 33.5

14. Cnidarians are carnivores
That use tentacles to capture prey
The tentacles are armed with cnidocytes
Unique cells that function in defense and the capture of prey
Tentacle
“Trigger”
Nematocyst
Coiled thread
Discharge
Of thread
Cnidocyte
Prey
Figure 33.6

15. The phylum Cnidaria is divided into four major classes
Table 33.1

16. Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa
(a) These colonial polyps are members of class Hydrozoa.
(b) Many species of jellies (class Scyphozoa), including the species pictured here, are bioluminescent. The largest scyphozoans have tentacles more than 100 m long dangling from a bell-shaped body up to 2 m in diameter.
(c) The sea wasp (Chironex fleckeri) is a member of class Cubozoa. Its poison, which can subdue fish and other large prey, is more potent than cobra venom.
(d) Sea anemones and other members of class Anthozoa exist only as polyps.
Figure 33.7a–d

17. Hydrozoans Most hydrozoans Alternate between polyp and medusa forms
Other polyps, specialized
for reproduction, lack
tentacles and produce tiny
medusae by asexual budding. 3
Some of the colony’s
polyps, equipped with tentacles,
are specialized for feeding. 2
The medusae
swim off, grow, and
reproduce sexually. 4
Feeding
polyp
Reproductive
Medusa
bud
ASEXUAL
REPRODUCTION
(BUDDING)
Gonad
MEIOSIS
FERTILIZATION
SEXUAL
Egg
Sperm
Developing
Portion of
a colony
of polyps
Mature
Planula
(larva)
Key
Haploid (n)
Diploid (2n)
1 mm
Zygote
Figure 33.8
A colony of
interconnected
polyps (inset,
LM) results
from asexual
reproduction
by budding. 1
The planula eventually settles
and develops into a new polyp. 6
The zygote develops into a
solid ciliated larva called a planula. 5

18. Scyphozoans In the class Scyphozoa
Jellies (medusae) are the prevalent form of the life cycle

19. In the class Cubozoa, which includes box jellies and sea wasps
Cubozoans
In the class Cubozoa, which includes box jellies and sea wasps
The medusa is box-shaped and has complex eyes

20. Class Anthozoa includes the corals and sea anemones
Anthozoans
Class Anthozoa includes the corals and sea anemones
Which occur only as polyps

21. 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 development

22. Members of phylum Platyhelminthes
Flatworms
Members of phylum Platyhelminthes
Live in marine, freshwater, and damp terrestrial habitats
Are flattened dorsoventrally and have a gastrovascular cavity
Although flatworms undergo triploblastic development
They are acoelomates

23. Flatworms are divided into four classes
Table 33.2

24. Turbellarian Turbellarians
Are nearly all free-living and mostly marine
Figure 33.9

25. The best-known turbellarians, commonly called planarians
Have light-sensitive eyespots and centralized nerve nets
Pharynx. The mouth is at the
tip of a muscular pharynx that
extends from the animal’s
ventral side. Digestive juices
are spilled onto prey, and the
pharynx sucks small pieces of
food into the gastrovascular
cavity, where digestion continues.
Digestion is completed within
the cells lining the gastro-
vascular cavity, which has
three branches, each with
fine subbranches that pro-
vide an extensive surface area.
Undigested wastes
are egested
through the mouth.
Ganglia. Located at the anterior end
of the worm, near the main sources
of sensory input, is a pair of ganglia,
dense clusters of nerve cells.
Ventral nerve cords. From
the ganglia, a pair of
ventral nerve cords runs
the length of the body.
Gastrovascular
cavity
Eyespots
Figure 33.10

26. Monogeneans and Trematode
Monogeneans and trematodes
Live as parasites in or on other animals
Parasitize a wide range of hosts

27. Trematodes that parasitize humans
Spend part of their lives in snail hosts
These larvae penetrate
the skin and blood
vessels of humans
working in irrigated
fields contaminated
with infected human
feces.
Asexual reproduction
within a snail results in
another type of motile
larva, which escapes from
the snail host.
Blood flukes reproduce
sexually in the human host.
The fertilized eggs exit the
host in feces.
The eggs develop in
water into ciliated
larvae. These larvae
infect snails, the
intermediate hosts.
Snail host
1 mm
Female
Male 5 2 3 4
Figure 33.11
Mature flukes live in the blood vessels of the human
intestine. A female fluke fits into a groove running
the length of the larger male’s body, as shown in
the light micrograph at right. 1

28. Most monogeneans
Are parasites of fish

29. Tapeworm Tapeworms Are also parasitic and lack a digestive system
Proglottids with
reproductive structures
200 µm
Hooks
Sucker
Scolex
Figure 33.12

30. Rotifers, phylum Rotifera
Are tiny animals that inhabit fresh water, the ocean, and damp soil

31. Rotifers are smaller than many protists
But are truly multicellular and have specialized organ systems
0.1 mm
Figure 33.13

32. Rotifers have an alimentary canal
A digestive tube with a separate mouth and anus that lies within a fluid-filled pseudocoelom
Rotifers reproduce by parthenogenesis
In which females produce more females from unfertilized eggs

33. Lophophorates: Ectoprocts, Phoronids, and Brachiopods
Lophophorates have a lophophore
A horseshoe-shaped, suspension-feeding organ bearing ciliated tentacles

34. Ectoprocts Are colonial animals that superficially resemble plants
Lophophore
Ectoprocts, such as this sea
mat (Membranipora
membranacea), are colonial
lophophorates.
(a)
Figure 33.14a

35. Phoronids
Are tube-dwelling marine worms ranging from 1 mm to 50 cm in length
Lophophore
In phoronids such as
Phoronis hippocrepia, the
lophophore and mouth
are at one end of an
elongated trunk.
(b)
Figure 33.14b

36. Brachiopods superficially resemble clams and other hinge-shelled molluscs
But the two halves of the shell are dorsal and ventral rather than lateral, as in clams
Lophophore
Brachiopods have a hinged shell.
The two parts of the shell are
dorsal and ventral.
(c)
Figure 33.14c

37. Members of phylum Nemertea
Nemerteans
Members of phylum Nemertea
Are commonly called proboscis worms or ribbon worms
Figure 33.15

38. The nemerteans unique proboscis
Is used for defense and prey capture
Is extended by a fluid-filled sac
Nemerteans also have a closed circulatory system
In which the blood is contained in vessels distinct from fluid in the body cavity

39. Most molluscs are marine
Concept 33.4: Molluscs have a muscular foot, a visceral mass, and a mantle
Phylum Mollusca
Includes snails and slugs, oysters and clams, and octopuses and squids
Most molluscs are marine
Though some inhabit fresh water and some are terrestrial
Molluscs are soft-bodied animals
But most are protected by a hard shell

40. All molluscs have a similar body plan with three main parts
A muscular foot
A visceral mass
A mantle

41. Visceral mass
Mantle
Foot
Coelom
Intestine
Gonads
cavity
Anus
Gill
Nerve
cords
Esophagus
Stomach
Shell
Radula
Mouth
Nephridium. Excretory organs
called nephridia remove metabolic
wastes from the hemolymph.
Heart. Most molluscs have an open circulatory
system. The dorsally located heart pumps
circulatory fluid called hemolymph through arteries
into sinuses (body spaces). The organs of the
mollusc are thus continually bathed in hemolymph.
The long digestive tract is
coiled in the visceral mass.
Radula. The mouth
region in many
mollusc species
contains a rasp-like
feeding organ
called a radula. This
belt of backward-
curved teeth slides
back and forth,
scraping and
scooping like a
backhoe.
The nervous
system consists
of a nerve ring
around the
esophagus, from
which nerve
cords extend.
Figure 33.16

42. Most molluscs have separate sexes
With gonads located in the visceral mass
The life cycle of many molluscs
Includes a ciliated larval stage called a trochophore

43. There are four major classes of molluscs
Table 33.3

44. Class Polyplacophora is composed of the chitons
Oval-shaped marine animals encased in an armor of eight dorsal plates
Figure 33.17

45. About three-quarters of all living species of molluscs
Gastropods
About three-quarters of all living species of molluscs
Belong to class Gastropoda
A land snail
(a)
A sea slug. Nudibranchs, or sea slugs, lost their shell
during their evolution.
(b)
Figure 33.18a, b

46. Most gastropods Slugs lack a shell
Are marine, but there are also many freshwater and terrestrial species
Possess a single, spiraled shell
Slugs lack a shell
Or have a reduced shell

47. The most distinctive characteristic of this class
Is a developmental process known as torsion, which causes the animal’s anus and mantle to end up above its head
Anus
Mantle
cavity
Stomach
Intestine
Mouth
Figure 33.19

48. Molluscs of class Bivalvia
Bivalves
Molluscs of class Bivalvia
Include many species of clams, oysters, mussels, and scallops
Have a shell divided into two halves
Figure 33.20

49. The mantle cavity of a bivalve
Contains gills that are used for feeding as well as gas exchange
Hinge area
Gut
Coelom
Heart
Adductor
muscle
Anus
Excurrent
siphon
Water
flow
Incurrent
Gill
Mantle
cavity
Foot
Palp
Mouth
Shell
Figure 33.21