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Chapter 33
Invertebrates
I. Parazoa
A. Phylum Porifera: Sponges Figure 33.2 (p. 647, ed.
6) – A sponge.
1. Non-moving (sessile) animals
2. No nerves or muscles (no tissue differentiation)
3. Mostly marine
4. Filter feeders: Collect food particles from water:
Remember cells involved in feeding - Figure 33.3 (p. 648, ed.
6; Fig 33.4, p. 642, ed. 7) – Anatomy of a sponge.
5. Most sponges are hermaphrodites. Hermaphrodites
function as both male and female in sexual reproduction by
producing eggs and sperm.
**All other animals are in the clade Eumetazoa (true
tissues).**
Figure 33.4 (p. 649, ed. 6; Fig. 33.5, p. 642, ed. 7) – Polyp and
medusa forms of cnidarians.
1. Radial symmetry with central digestive (gastrovascular)
cavity.
2. One opening in the gastrovascular cavity serves as both
mouth and anus.
3. Survive as carnivores.
4. Phylum name comes from specialized cells called
cnidocytes. Cnidocytes are stinging cells used for defense
and to capture prey.
Figure 33.5 (p. 649) – A cnidocyte of a hydra.
B. Phylum Ctenophora: Comb jellies
Figure 33.8 (p. 651) – A ctenophore, or comb jelly.
1. Resemble cnidarian medusas.
2. Use cilia for locomotion.
III. Protostomia: Lophotrochozoa
A. Phylum Platyhelminthes: Flatworms
1. Sizes range from microscopic up to 20 meters long
(tapeworms).
2. Many are parasites.
Table 33.2 (p. 652, ed. 6; p. 646, ed. 7) – Classes of Phylum
Platyhelminthes
3. There are four classes of flatworms; know
the common names.
a. Turbellarians
i. Mostly free-living (non-parasitic)
ii. Feed on small animals, dead animals
iii. Very flat for O2 exchange. They have no gas exchange
organs.
Figure 33.10 (p. 653) – Anatomy of a planarian.
b,c. Monogenea/Trematoda
i. Live as parasites
ii. Trematodes parasitize vertebrates. For example, humans
but often with intermediate hosts.
Figure 33.11 (p. 653, ed. 6; p. 647, ed. 7) – The life history of a
blood fluke (Schistosoma mansoni).
d. Cestoidea – Tapeworms
i. Live as parasites
ii. Head contains suckers and hooks that lock onto the
intestinal lining of the host.
iii. The rest of the body is mostly units called proglottids that
are sex organs.
iv. Eggs transferred to new hosts by consuming fecal
contaminated water.
Figure 33.12 (p. 654) – Anatomy of a tapeworm.
B. Phylum Rotifera: Rotifers
1. Aquatic
2. Sizes range from 0.5 to 2 mm
3. Complete digestive tract
Figure 33.13 (p. 654) – A rotifer.
C. Lophophorate Phyla: Bryozoans (Ectoprocts, ed.
7), Phoronids, and Brachiopods
1. All of these groups have a lophophore. The lophophore is
a horseshoe shaped fold near the mouth that is surrounded
by ciliated tentacles.
2. Bryozoans resemble mosses.
- They have a hard exoskeleton.
- They are important as reef builders.
Figure 33.14a (p. 655; p. 649, ed. 7) – A
bryozoan (Lophophorates).
3. Phoronids are marine worms.
4. Brachiopods resemble clams.
Figure 33.14b (p. 655) – A brachiopod.
D. Phylum Nemertea: Proboscis worms
1. Up to 30 meters in length
2. These worms have a hydraulically-operated
proboscis that is used to capture prey.
Figure 33.15
E. Phylum Mollusca: Mollusks
1. Snails, clams, octopi, squids, oysters
2. There are at least 150,000 known species
3. All mollusks have similar body plans:
a. Muscular foot
b. Visceral mass with organs
c. Mantle that secretes the shell
b. Gastropoda – Produce shells into which the animal
retreats when threatened. Example: Snail
Figure 33.19 (p. 657) – Gastropods.
c. Bivalvia – Shells divided into two parts. Gills are used for
feeding and gas exchange. Example: Clams, oysters
Figure 33.21 (p. 658) – Anatomy of a clam.
d. Cephalopoda – Rapid movement; well-developed nervous
systems. Example: Octopus, squid, nautilus
Figure 33.22 (p. 658) – Cephalopods.
F. Phylum Annelida: Segmented worms
1. Sizes range from 1 mm to 3 meters in length.
2. Each segment contains a pair of excretory tubes called
metanephridia.
3. Annelids are hermaphrodites that cross-fertilize.
4. Three classes:
a. Oligochaeta – earthworms
b. Polychaeta – mostly marine
c. Hirudinea – leeches
IV. Protostomia: Ecdysozoa
A. Phylum Nematoda: Roundworms
1. Non-segmented
2. Some are important parasites of animals.
- Pinworms and hookworms  e.g., Trichinella obtained by
eating undercooked pork; juveniles infect all body organs
and tissues.
B. Phylum Arthropoda: Arthropods
1. Crustaceans, spiders, insects
2. Hard exoskeleton, segmented bodies, jointed appendages
3. Arthropods are the most successful of all animal phyla
based on diversity, distribution, and numbers.
4. Nearly one million species identified so far, mostly insects.
5. The exoskeleton, or cuticle, is composed of protein and
chitin.
6. Molting of the cuticle is called ecdysis.
7. Extensive cephalization.
8. Arthropods have open circulatory systems in which a
heart pumps hemolymph through short arteries and into
open spaces called sinuses.
9. Aquatic members have gills for gas exchange. Terrestrial
members have a tracheal system  branched tubes leading
from the surface throughout the body.
Figure 33.26 (p. 663) – External anatomy of an arthropod.
10. Four evolutionary lineages:
a.
b.
c.
d.
Trilobites – extinct
Chelicerates – horseshoe crabs, spiders
Uniramians – centipedes, millipedes, insects
Crustaceans – crabs, lobsters, barnacles
11. From these lineages arose five major classes of
arthropods:
Table 33.5
d. Insecta – insects 
i. Outnumber all other forms of animals.
ii. Evolved flight during Carboniferous period. Flight was
followed by an explosion of diversity.
iii. Coevolution of flowering plants and insects.
iv. Wings are extensions of the cuticle.
v. Waste is removed from hemolymph by excretory organs
called malpighian tubes.
vi. Nervous system has a pair of ventral nerve chords that
join in the head to form a cerebral ganglion (brain) that is
close to the sensory organs in the head (cephalization).
Note: Many insects undergo metamorphosis during their
development.
Incomplete metamorphosis is a process whereby the young
look like the adults, but have different body proportions.
Complete metamorphosis is a process where the larval
stages are specialized for eating. This stage is called a
larva, maggot or grub. The adult stage is specialized for
reproduction and dispersal (e.g. flight). The process of
metamorphosis occurs during a pupal stage.
ORDER
Blattodea
Coleoptera
Dermaptera
Diptera
Hemiptera
Hymenoptera
APPROXIMATE
NUMBER OF
SPECIES
4,000
350,000
MAIN CHARACTERISTICS
EXAMPLES
Cockroaches have a dorsoventrally flattened body, with legs
modified for rapid running. Forewings, when present, are
leathery, whereas hind wings are fanlike. Fewer than 40 cockroach species live in houses; the rest exploit habitats ranging
from tropical forest floors to caves and deserts.
German
cockroach
Beetles comprise the most species-rich order of insects. They
have two pairs of wings, one of which is thick and leathery, the
other membranous. They have an armored exoskeleton and
mouthparts adapted for biting and chewing. Beetles undergo
complete metamorphosis.
Japanese
beetle
1,200
Earwigs are generally nocturnal scavengers. While some
species are wingless, others have two pairs of wings, one of
which is thick and leathery, the other membranous. Earwigs
have biting mouthparts and large posterior pincers. They undergo incomplete metamorphosis.
Earwig
151,000
Dipterans have one pair of wings; the second pair has become
modified into balancing organs called halteres. Their head is
large and mobile; their mouthparts are adapted for sucking,
piercing, or lapping. Dipterans undergo complete metamorphosis. Flies and mosquitoes are among the best-known dipterans,
which live as scavengers, predators, and parasites.
85,000
125,000
Hemipterans are so-called “true bugs,” including bed bugs,
assassin bugs, and chinch bugs. (Insects in other orders are
sometimes erroneously called bugs.) Hemipterans have two
pairs of wings, one pair partly leathery, the other membranous.
They have piercing or sucking mouthparts and undergo
incomplete metamorphosis.
Horsefly
LeafFooted
bug
Ants, bees, and wasps are generally highly social insects. They
have two pairs of membranous wings, a mobile head, and
chewing or sucking mouthparts. The females of many species
have a posterior stinging organ. Hymenopterans undergo complete metamorphosis.
Cicada-killer wasp
Isoptera
Figure 33.37
2,000
Termites are widespread social insects that produce enormous
colonies. It has been estimated that there are 700 kg of
termites for every person on Earth! Some termites have two
pairs of membranous wings, while others are wingless. They
feed on wood with the aid of microbial symbionts carried in
specialized chambers in their hindgut.
Termite
ORDER
Lepidoptera
APPROXIMATE
NUMBER OF
SPECIES
120,000
5,000
Odonata
EXAMPLE
MAIN CHARACTERISTICS
Butterflies and moths are among the best-known insects. They
have two pairs of wings covered with tiny scales. To feed, they
uncoil a long proboscis. Most feed on nectar, but some species
feed on other substances, including animal blood or tears.
Swallowtail
butterfly
Dragonflies and damselflies have two pairs of large, membranous wings. They have an elongated abdomen, large, compound
eyes, and chewing mouthparts. They undergo incomplete metamorphosis and are active predators.
Dragonfly
Orthoptera
13,000
Grasshoppers, crickets, and their relatives are mostly herbivorous. They have large hind legs adapted for jumping, two
pairs of wings (one leathery, one membranous), and biting or
chewing mouthparts. Males commonly make courtship sounds
by rubbing together body parts, such as a ridge on their hind
leg. Orthopterans undergo incomplete metamorphosis.
Katydid
Phasmida
Phthiraptera
Siphonaptera
2,600
2,400
2,400
Stick insects and leaf insects are exquisite mimics of plants. The
eggs of some species even mimic seeds of the plants on which the
Insects live. Their body is cylindrical or flattened dorsoventrally.
They lack forewings but have fanlike hind wings. Their
mouthparts are adapted for biting or chewing.
Commonly called sucking lice, these insects spend their entire
life as an ectoparasite feeding on the hair or feathers of a single
host. Their legs, equipped with clawlike tarsi, are adapted for
clinging to their hosts. They lack wings and have reduced eyes.
Sucking lice undergo incomplete metamorphosis.
Stick insect
Human
Body
louse
Fleas are bloodsucking ectoparasites on birds and mammals.
Their body is wingless and laterally compressed. Their legs are
modified for clinging to their hosts and for long-distance
jumping. They undergo complete metamorphosis.
Flea
Thysanura
450
Silverfish are small, wingless insects with a flattened body and
reduced eyes. They live in leaf litter or under bark. They can also
infest buildings, where they can become pests.
Silverfish
Trichoptera
Figure 33.37
7,100
The larvae of caddisflies live in streams, where they make houses
from sand grains, wood fragments, or other material held together by silk. Adults have two pairs of hairy wings and chewing
or lapping mouthparts. They undergo complete metamorphosis.
Caddisfly
e. Crustacea – crabs, lobsters
i. 40,000 species.
ii. Includes krill eaten by whales.
V. Deuterostomia
A. Phylum Echinodermata: Echinoderms
1. Water vascular systems – network of hydraulic canals
used for locomotion, feeding, and gas exchange. It
extends into tube feet that are used for locomotion and
feeding.
Figure 33.38 (p. 674) – Anatomy of a sea star.
2. Echinoderms appear to be radial, but are bilateral 
shows up in larval stages.
B. Phylum Chordata: Chordates
1. Chordates belong to this category but will be
discussed in the next chapter.