Chapter 23
Section 1 –
Section 2 –
Section 3 –
Section 4 –
Section 5 –
Section 6 –
Section 7 –
Section 8 –
Section 9 –
Invertebrate Diversity
Diverse animals share several key
characteristics
Sponges are relatively simple animals with porous
bodies
Cnidarians are radial animals with stinging cells
Flatworms are the simplest bilateral animals
Roundworms and rotifers have complete digestive
tracts
Annelida are segmented worms
Mollusks show diverse variations on a common
body form
Echinoderms have spiny skin and a water vascular
system
Animal diversity “exploded” during the Cambrian
period
The Body of a Flatworm

Flatworms (phylum Platyhelminthes) are bilaterally
symmetrical.
Definition: member of a group of small, leaflike or
ribbonlike invertebrates that includes planarians
 Definition: body plan in which an animal can be divided
into two equal sides
Flatworms have mirror-image left and right sides, a distinct
head (anterior end), a distinct tail (posterior end), a
back side (dorsal), a bottom side (ventral), and two side
surfaces (lateral).
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Flatworms are the simplest animals to have three
tissue layers: the ectoderm, mesoderm, and
endoderm.
During an animal’s early development:
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THE ECTODERM: develops into the body covering.
THE MESODERM: develops into an internal tissue-filled
region.
THE ENDODERM: develops into the digestive sac.
Similar to cnidarians, flatworms have a digestive
cavity – food enters and wastes exit from one opening
located on the ventral surface.
 When the animal is feeding, a muscular tube projects
through the mouth and sucks in food.

Flatworms can move in several ways:
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They use cilia on its ventral surface to slide about in search
for food.
They have muscles that enable it to twist and turn.
Flatworms are aware of their surroundings by:


the pair of eyespots that detect light.
the side flaps that function mainly for smell.
The Diversity of Flatworms

The 20,000 known species are divided into three
classes:
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Class Turbellaria: mostly free-living and marine
(planarians)
Class Trematoda: parasites that absorb nutrients from the
body fluids of a living host (blood fluke)
Class Cestoidea: parasites that live in the digestive tracts
of vertebrates (tapeworms)
Chapter 23
Section 1 –
Section 2 –
Section 3 –
Section 4 –
Section 5 –
Section 6 –
Section 7 –
Section 8 –
Section 9 –
Invertebrate Diversity
Diverse animals share several key
characteristics
Sponges are relatively simple animals with porous
bodies
Cnidarians are radial animals with stinging cells
Flatworms are the simplest bilateral animals
Roundworms and rotifers have complete digestive
tracts
Annelida are segmented worms
Mollusks show diverse variations on a common
body form
Echinoderms have spiny skin and a water vascular
system
Animal diversity “exploded” during the Cambrian
period
The Body of a Roundworm

Most roundworms, or nematodes (phylum
Nematoda), range in length from less than 1 mm
to more than 7 meters.
Definition: member of a group of cylindrical
invertebrates with pointed heads and tapered tails
The largest roundworms are parasites found in
whales.
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


Like flatworms, roundworms have three tissue
layers.
One characteristic that makes roundworms (and
annelids) different from flatworms (and cnidarians) is the
presence of a complete digestive tract.

Definition: continuous digestive tube with a separate
mouth and anus

In animals with a digestive tract, the anterior
region of the tract churns and mixes food with
enzymes for digestion – the posterior region
absorbs nutrients from the digested food and
disposes of wastes.
The Diversity of Roundworms

Roundworms are among the most numerous on
Earth, totaling about 15,000 known species.

Roundworms live almost every place there is rotting
organic matter and play a roles as important
decomposers on the bottom of lakes and oceans.
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
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Other roundworms thrive as parasites in the moist tissues
of plants and in the body fluids and tissues of animals.
Free-living roundworms (non-parasitic) roundworms
are the most abundant.
Many species of parasitic roundworms attack the
roots of plants and tissues of animals – humans can
be hosts to at least 50 species of roundworms parasites.
 One disease that can affect humans is called trichinosis 
humans acquire this disease by eating undercooked pork
or other meat that is infected with worms.
 Roundworm parasites of animals are known by several
common names such as hookworm, pinworm, and
threadworm (names based on worm’s appearance).
Chapter 23
Section 1 –
Section 2 –
Section 3 –
Section 4 –
Section 5 –
Section 6 –
Section 7 –
Section 8 –
Section 9 –
Invertebrate Diversity
Diverse animals share several key
characteristics
Sponges are relatively simple animals with porous
bodies
Cnidarians are radial animals with stinging cells
Flatworms are the simplest bilateral animals
Roundworms and rotifers have complete digestive
tracts
Annelida are segmented worms
Mollusks show diverse variations on a common
body form
Echinoderms have spiny skin and a water vascular
system
Animal diversity “exploded” during the Cambrian
period
The Body of an Annelid

Earthworms and other segmented worms (phylum
annelida) are called annelids.
Definition: segmented worm
Annelida  “little rings”
Segmentation is an advantage because each segment has its
own muscles, allowing shortening and lengthening of the
body for movement.
Within each segment is a dense cluster of nerve cells and
waste-excreting organs.
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Annelids have a distinct head and tail and it’s body
segments are all very similar.

The digestive tract is not segmented and runs the length
of the animal, along with the nerve cord and two main
blood vessels.

FIGURE 23-12

Annelids have a closed circulatory system.
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Definition: blood transport system in which blood
remains enclosed in vessels; nutrients, oxygen, and
wastes diffuse through vessel walls
On the underside of a
segmented worm are bristles
– these bristles aid in movement.
FIGURE 23-15

Annelids are bilaterally symmetrical and have three
tissue layers.
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
Bilateral symmetry  when divided in half, it is identical on
both sides (mirror image)
The tissue layers are organized differently in each of the
three groups of worms.
FLATWORMS are examples of acoelomates.
 Definition: animal lacking a body cavity
 ROUNDWORMS each have a pseudocoelom.
 Definition: fluid-filled internal space that is in direct
contact with the wall of the digestive tract
 ANNELIDS each have a coelom.
 Definition: fluid-filled body cavity completely lined by
a layer of mesoderm cells

The Habitat of Annelids

Segmented worms live in the soil, fresh water, and the
sea – everywhere except frozen soil and dry sand.
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The soil is the worms main source of oxygen – it diffuses
into the earthworm through the skin.
DURING THE DAY: The cool, moist soil provides protection
for the worm.
AT NIGHT: They come to the surface and stay close to their
burrows.
The Reproduction of Annelids

Segmented worms are hermaphrodites (bodies contain
male and female sex organs).

During mating, two worms exchange sperm  the sperm
fertilizes the eggs contained in the bodies of each worm
 the fertilized eggs are expelled in the soil  two-three
weeks later, worms hatch from the eggs.
The Diversity of Annelids

There are about 15,000 known species of
annelids, grouped into three classes.
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Class Polychaeta: includes species that scavenge for
food on the ocean floor (sandworms)
Class Oligochaeta: includes species who tunnel
through the soil helping air to circulate in it (earthworms)
Class Hirudinea: includes parasitic leeches