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

2. Symmetry Animals can be categorized
According to the symmetry of their bodies, or lack of it

3. Some animals have radial symmetry
Like in a flower pot
Radial symmetry. The parts of a radial animal, such as a sea anemone (phylum Cnidaria), radiate from the center. Any imaginary slice through the central axis divides the animal into mirror images.
(a)
Figure 32.7a

4. Some animals exhibit bilateral symmetry
Or two-sided symmetry
Bilateral symmetry. A bilateral animal, such as a lobster (phylum Arthropoda), has a left side and a right side. Only one imaginary cut divides the animal into mirror-image halves.
(b)
Figure 32.7b

5. Bilaterally symmetrical animals have
A dorsal (top) side and a ventral (bottom) side
A right and left side
Anterior (head) and posterior (tail) ends
Cephalization, the development of a head

6. Tissues Animal body plans Tissues
Also vary according to the organization of the animal’s tissues
Tissues
Are collections of specialized cells isolated from other tissues by membranous layers

7. Body Cavities In triploblastic animals
A body cavity may be present or absent

8. A true body cavity Is called a coelom and is derived from mesoderm
Figure 32.8a
Coelom
Body covering
(from ectoderm)
Digestive tract
(from endoderm)
Tissue layer
lining coelom
and suspending
internal organs
(from mesoderm)
Coelomate. Coelomates such as annelids have a true coelom, a body cavity completely lined by tissue derived from mesoderm.
(a)

9. A pseudocoelom
Is a body cavity derived from the blastocoel, rather than from mesoderm
Pseudocoelom
Muscle layer
(from
mesoderm)
Body covering
(from ectoderm)
Digestive tract
Pseudocoelomate. Pseudocoelomates such as nematodes have a body cavity only partially lined by tissue derived from mesoderm.
(b)
Figure 32.8b

10. Organisms without body cavities
Are considered acoelomates
Body covering
(from ectoderm)
Tissue-
filled region
(from
mesoderm)
Digestive tract
(from endoderm)
Acoelomate. Acoelomates such as flatworms lack a body cavity between the digestive tract and outer body wall.
(c)
Figure 32.8c

11. Protostome and Deuterostome Development
Based on certain features seen in early development
Many animals can be categorized as having one of two developmental modes: protostome development or deuterostome development

12. 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

13. 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

14. 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

15. 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

16. Parazoa – Phylum Porifera
Sponges
Loosely organized and lack tissues
Multicellular with several types of cells
8,000 species mostly marine
No apparent symmetry
Adults sessile, larvae free-swimming

17. Water drawn through pores (ostia) into spongocoel
Flows out through osculum
Choanocytes line spongocoel
Trap and eat small particles and plankton
Reproduce sexually and asexually
Most hermaphrodites producing eggs and sperm

19. Sponges are sessile and have a porous body and choanocytes
Sponges, phylum Porifera
Live in both fresh and marine waters
Lack true tissues and organs

20. 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

21. 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

22. Radiata – Phylums Cnidaria and Ctenophora
Radial symmetry
Mostly marine
Only 2 embryonic germ layers – diploblastic
Ectoderm and endoderm
Mesoglea connects 2 layers
Gastrovascular cavity for extracellular digestion
True nerve cells arranged in nerve net
No central control organ

23. Phylum Cnidaria 2 different body forms Cnidocytes contain nemotocysts
Sessile polyp – tubular body with tentacles surrounding opening (mouth and anus)
Motile medusa – umbrella-shaped body with a mouth on the underside surrounded by tentacles
Cnidocytes contain nemotocysts
Hairlike trigger – cnidocil
Some sticky while other sting
Simple muscles and nerves
Not true muscles with mesoderm

25. 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

26. 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

27. 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

28. 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

29. Cnidarians are carnivores The tentacles are armed with cnidocytes
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

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

31. 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

32. 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

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

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

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

36. 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

37. 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

38. Flatworms are divided into four classes
Table 33.2

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

40. Phylum Platyhelminthes
Flatworms
Lack a specialized respiratory or circulatory system to transport gases
Respire by diffusion
Among first animals with active predatory lifestyle
Bilaterally symmetrical with a head
First with 3 embryonic germ layers – triploblastic
Mesoderm key innovation – led to more sophisticated organs
Acoelomate – lacking fluid-filled cavity

42. Digestive system incomplete
Distinct excretory system with protonephridia and flame cells
Light sensitive eyespots or ocelli
Cerebral ganglia receive input
Retain nerve net with beginning of more centralized nervous system
Sexual or asexual reproduction
Most hermaphroditic but do not self fertilize
4 classes – Turbellaria, Monogenea, Trematoda and Cestoda

43. Turbellaria – Free-living, Planaria Monogenea – Fish flukes
Cestoda – Tapeworms, parasitic
2 separate host species in life cycle
Trematoda – Flukes, parasitic
More complex life cycle with multiple hosts
Chinese liver fluke, Clonorchis sinensis
Blood flukes, Schistosoma spp., most common parasitic trematode infecting humans

44. 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

45. Concept 33.5: Annelids are segmented worms Annelids
Have bodies composed of a series of fused rings

46. The phylum Annelida is divided into three classes
Table 33.4

47. Oligochaetes Oligochaetes (class Oligochaeta)
Are named for their relatively sparse chaetae, or bristles made of chitin
Include the earthworms and a variety of aquatic species

48. Phylum Annelida Rings are distinct segments separated by a septum
Segmentation has advantages
Repetition of components provides backup
Coelom acts as hydrostatic skeleton
Permits specialization
Double transport system
Circulatory system and coelomic fluid carries nutrients, wastes and respiratory gases

50. Digestive system complete and unsegmented
Sexual reproduction involves 2 individuals (sometimes separate sexes other hermaphroditic) with internal fertilization
Asexual reproduction by fission
15,000 species
All annelids except leeches have setae on each segment
3 classes – Polychaeta, Oligochaeta and Hirudinea

51. Polychaeta – marine worms
Most species rich, many long setae
Oligochaeta – terrestrial and freshwater worms (earthworms)
Role in conditioning soil through castings
Hirudinea – leeches
Primarily freshwater, hirudin (anticoagulant), may be used in reattachment surgeries, generally external parasites

52. Ecdysozoa
Separation from Lophotrochozoa supported by molecular data and morphological characteristic
Ecdysis or molting
All ecdysozoans possess a cuticle for support and protection
Developmental options – metamorphosis

53. Earthworms eat their way through the soil, extracting nutrients as the soil moves through the alimentary canal
Which helps till the earth, making earthworms valuable to farmers

54. Giant Australian earthworm
Anatomy of an earthworm
Mouth
Subpharyngeal
ganglion
Pharynx
Esophagus
Crop
Gizzard
Intestine
Metanephridium
Ventral
vessel
Nerve
cords
Nephrostome
Dorsal
Longitudinal
muscle
Circular
Epidermis
Cuticle
Septum
(partition
between
segments)
Anus
Each segment is surrounded by longitudinal muscle, which in
turn is surrounded by circular muscle. Earthworms coordinate
the contraction of these two sets of muscles to move (see
Figure 49.25). These muscles work against the noncompressible
coelomic fluid, which acts as a hydrostatic skeleton.
Coelom. The coelom
of the earthworm is
partitioned by septa.
Metanephridium. Each
segment of the worm
contains a pair of
excretory tubes, called
metanephridia, with
ciliated funnels, called
nephrostomes. The
metanephridia remove
wastes from the blood
and coelomic fluid
through exterior pores.
Tiny blood vessels are
abundant in the earthworm’s
skin, which functions as its
respiratory organ. The blood
contains oxygen-carrying
hemoglobin.
Ventral nerve cords with segmental ganglia.
The nerve cords penetrate the septa and run
the length of the animal, as do the digestive
tract and longitudinal blood vessels.
The circulatory system, a network of vessels,
is closed. The dorsal and ventral vessels are linked
by segmental pairs of vessels. The dorsal vessel
and five pairs of vessels that circle the esophagus
of an earthworm are muscular and pump blood
through the circulatory system.
Cerebral ganglia. The
earthworm nervous system
features a brain-like pair of
cerebral ganglia above and
in front of the pharynx. A ring
of nerves around the pharynx
connects to a subpharyngeal
ganglion, from which a fused
pair of nerve cords runs
posteriorly.
Chaetae. Each segment
has four pairs of
chaetae, bristles that
provide traction for
burrowing.
Many of the internal
structures are repeated
within each segment of
the earthworm.
Giant Australian earthworm
Clitellum
Table 33.23

55. Most molluscs are marine Molluscs are soft-bodied animals
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

56. Phylum Mollusca Over 100,000 species
Soft body with, in many species, protective external shell
Body has 3 parts
Foot, visceral mass and mantle
Coelom confined to small area around heart
Open circulatory system
Metanephridia
Radula – unique tongue-like organ

58. Most shells complex 3 layered and secreted by mantle
Separate sexes although some hermaphroditic
External fertilization – some internal (key to snails colonizing land)
Trocophore larvae develops into veliger with rudimentary foot, shell and mantle
8 classes with 4 common
Polyplacophorans, gastropods, bivalves and cephalopods

60. Polyplacophorans – chitons Gastropods – snails, slugs and nudibranchs
Largest class, shells can be reduced or lost, most marine or freshwater but some colonized land
Bivalves – clams, mussels, oysters
Cephalopods – octopuses, squids, nautiluses
Most morphologically complex, fast-swimming marine predators, closed circulatory system
Beaklike jaw, only nautilus has external shell, some have foot modified into muscular siphon for propulsion

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

62. 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

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

64. There are four major classes of molluscs
Table 33.3

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

66. 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