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Chapter 33: Invertebrates
-Invertebrates = animals without backbones
-Invertebrates account for 95% of known animal species and occupy almost every habitat on earth
I. Sponges are sessile and have porous body and choanocytes
 Phylum Porifera, about 9,000 known species
 Parazoans that lack true tissues. They have two distinct cell layers that are separated by a gelatinous
region called the mesohyl.
 Closest lineage to protists???
 Sessile
 Height ranges from 1 cm. To 2 meters
 The body is a sac with holes (pores. . .gives the name porifera)
 Water is drawn into the pores and enters the central cavity called the spongocoel
 The water leaves the spongocoel through the osculum (large cavity)
 Most are suspension feeders, or ‘filter feeders’ and collect food from water filtered through
flagellated cells called choanocytes (collar cells).
 Water is pulled into the sponges by the movement of flagella. The food particles are caught by the
chaonocytes’ collar. The chaonocytes phagocytose the food and the ameobocyte moves the food
where it needs to go. FIG. 33.4, pg. 642
 Most are hermaphrodites-meaning that each individual functions as both male and female in sexual
reproduction by producing sperm and eggs.
II. Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes
 Phylum Cnidaria (hydras, jellies, sea anemones, and coral animals)
 Eumetazoans-animals with true tissues
 Diploblastic with radial symmetry
 Lack mesoderm
 Some are sessile and others have floating forms
 Basic body is a sac with a central digestive compartment called the gastrovascular cavity.
 A single opening into the G.C. acts as both a mouth and anus
 Two body plans FIG. 33.5, pg. 643
1. Polyps: cylindrical forms that stick to a substrate with the mouth/anal opening on the upper
side. Tentacles face upward. EX. Hydras and Sea Anemones
2. Medusa: flattened version of the polyp with a non-attached mouth/anus opening toward the
underside. Tentacles face down. EX. Jellies and Medusas
 Carnivores that capture prey by tentacles
 Tentacles are arranged in a ring around the mouth. The tentacles have cnidocytes which capture/sting
prey FIG. 33.6, pg. 643. The cnidocytes contain cnidae. Cnidae called nematocysts are stinging
capsules, while other cnidae have long threads that stick to small prey that bump into the tentacles
 Sensory receptors are found all around the body for stimulus responses
 Contractile tissues and nerves occur in their simplest forms
 The gastrovascular cavity acts as a hydrostatic skeleton which allows the cells to contract and change
shape.
 Life cycles are both sexual and asexual (Figure 33.8, page 645)

Four classes of Cnidaria (Figure 33.7, page 644):
1. Hydrozoa-both polyp and medusa forms (Portuguese man-of-war, hydras, Obelia, etc)
2. Scyphozoa-medusa form prevails, reduced polyp (jellies, sea nettles, etc)
3. Cubozoans-have a box-shaped medusa stage and complex eyes in the fringe of their
medusae (box jellies, sea wasps)
4. Anthozoa-only occur as polyps (sea anemones and corals). Coral reefs are critical habitats
for ocean life and are being destroyed by pollution, global warming, and over-fishing
B. Phylum Ctenophore (comb jellies) FIG. 33.7, pg. 604
 Largest animals to use cilia for movement
 Have long tentacles with colloblasts (sticky structures that capture food)
III. Most animals have bilateral symmetry
 Clade Bilateria consists of animals with bilateral symmetry and triploblastic development.
A. Phylum Platyhelminthes (Flatworms)
 ≈ 20,000 species
 Acoelomates-all cells are close to surrounding water for gas exchange and waste removal
 Free living (in marine, freshwater, and damp terrestrial environments) and Parasitic
 Thin or Flat between the dorsal and ventral surfaces (flatworms)
 Range in size from being microscopic to over 20 meters long
 Only 1 opening for the gastrovascular cavity
 Four Classes:
1. Turbellaria (free-living flatworms)
 Mostly marine, some freshwater
 A common example is the Planarian, a carnivore that preys on small or dead animals. FIG.
33.9, pg. 646 and Figure 33.10, pg. 647)
 Ciliated cells called flame cells waft fluid through ducts to maintain osmotic balance
 Move by cilia sliding on secreted mucus
 Have eyespots to detect light and lateral flaps to detect specific chemicals
 Reproduce asexually through regeneration and sexually
2. Monogenea
 Marine and freshwater parasites, usually use fish for hosts
 Simple life cycle with a ciliated free-swimming larva that infest the host
 Have a tough covering to protect damage from their hosts
3. Trematodes
 Parasites, usually on vertebrate hosts
 Have a tough covering
 Complex life cycle with both sexual and asexual stages. FIG. 33.11, pg. 647. often require an
intermediate host for larval development
 200 million people worldwide are infected with blood flukes
4. Cestoidea (tape worms) FIG. 33.12, pg. 648
 Parasitic, adults live mostly on vertebrates, including humans
 The head (scolex) is lined with suckers and hooks that lock into the hosts’ intestines.
 Lack a gastrovascular cavity and absorb nutrients from the host by diffusion across the
tapeworms body surface
 The proglottids are long units of reproductive structures (sacs of sex organs with thousands of
eggs). The mature proglottids leave the hosts through feces.
 Humans acquire the larvae by eating undercooked meat
The Pseudocoelomates (body cavities not completely lined by mesoderm tissue)
A. Phylum Rotifera “wheel-bearers” (Figure 33.13, page 648)
 There are about 1,800 species of rotifers
 All are less than 2 mm long
 Small animals mainly found in freshwater ecosystems (but also in the ocean and damp soil)
 Have a complete digestive tract with separate mouth and anus, called the alimentary canal
 Fluid in pseudocoelom serves for support, transport, and ‘circulation’
 Have a crown of cilia that draws a vortex of water into their mouth
 Some reproduce by parthogenesis, where females produce females from unfertilized eggsoccasionally females produce ‘simplified males’
Lophophorates
 Have a true coelom
 Have a lophophore (a horseshoe shaped or circular crown of ciliated tentacles that
surround the mouth) SEE Figure 33.14, page 649.
 As cilia draw water toward the mouth, the tentacles trap food particles
 There are three phyla of lophophores:
A. Ectoprocts: colonial animals that look like plants (aka bryozoans). The colony is encased in a
hard exoskeleton with pores. Mostly marine (some found in lakes and rivers) and sessile.
B. Phoronids: tube-dwelling marine worms ranging from 1 mm to 50 cm in length. Some species
live in the sand within tubes made of chitin
C. Brachiopods: “lamp shells” resemble clams and other hinge-shelled molluscs. The two halves
of the shell are dorsal and ventral (rather than lateral like clams). All are marine and attach to
the seafloor by a stalk.
Nemerteans
 Phylum Nemertea (Ribbon Worms or Proboscis Worms) SEE Figure 33.15, page 649
 Structurally acoelomates, but contains a small fluid-filled sac that may be a reduced version of a
coelom
 The sac and fluid hydraulically operate an extensible proboscis (which shoots a toxin to its prey)
 ≈ 900 species, mainly marine (a few freshwater and damp soil species exist)
 1 mm to several meters
 Similar to flatworms with excretory, sensory and nervous systems
 Differ from flatworms in that they have a complete digestive tract (alimentary canal) and a closed
circulatory system in which the blood is contained in vessels (but they have no heart)
IV. Molluscs have a muscular foot, a visceral mass, and a mantle
 Coelomates and Protostomes
 Phylum Mollusca (Snails, slugs, oysters, clams, octopi, and squids)
 ≈ 150,000 species
 Mostly marine (a few are found in fresh water and on land)
 Usually soft-bodied with hard shells of calcium carbonate. Some molluscs shells are internal and very
reduced in size, like the slug and squid.
 Three main body parts FIG. 33.16, pg. 650
1. Foot-Used for movement
2. Visceral mass-Contains most internal organs
3. Mantle-A fold of tissue that drapes over the V.M. and secretes a shell
 Occasionally, the mantle extends past the V.M. and produces the mantle cavity which houses the
gills, anus, and excretory pores
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


A strap-like organ called a radula is used to scrape food
The life cycle may include a trochophore or ciliated larva
No segments
There are 8 classes of Molluscs, four important one are:
1. Polyplacophora (chitons)-oval shaped bodies and an 8 plate shell. They live on rocks near ocean
shore. FIG 33.17, pg. 651
2. Gastropoda (snails and slugs)-the largest class. They have torsion, which means that during
embryonic development, its V.M. rotates 180°, causing the animal’s anus and mantle cavity to
wind up above its head. They usually have a spiral, conical shell and move at a ‘snails pace’:)
FIG. 33.19, pg 651
3. Bivalva (clams, oysters, mussels, and scallops)-shells are divided into two halves which are
hinged together. Adductor muscles draw these sides tightly together. They are suspension
feeders that trap food in the mucus of their gills. FIG 33.20 and 33.21, pg 652
4. Cephalopoda (octopuses and squids)-body is built for speed because they are hungry carnivores.
They use tentacles to grasp prey and they have beaklike jaws to bite prey and then they inject
them with poison. The foot is modified into a muscular excurrent siphon which is used for
movement. The shell is reduced or missing (the Nautilus is the only one with a shell).
Cephalopods have a closed circulatory system and the most complex of invertebrate brains. Most
are less than 75 cm long, although the search for the giant squid continues. FIG. 33.22 page 653.
V. Annelids are segmented worms
 Phylum Annelid “little rings” (segmented worms)
 Key features include a coelom and segmentation
 ≈ 15,000 species, ranging in size from 1 mm to 3 m
 Live in the sea, freshwater habitats, and soil
 The digestive tract, blood vessels and nerve cords run the length of the body
 The digestive system has a pharynx, and esophagus, a crop, a gizzard, and intestine.
 Each segment has excretory tubes called metanephridia that remove wastes
 Have brain-like ganglia
 Hermaphrodites that cross fertilize by exchanging sperm
 Three classes
1. Oligochaeta-(earthworms)-eat their way through the soil. They have a reduced head and no
parapodia (false feet). They till and fertilize soil. FIG 33.23, pg. 654
2. Polychaeta-each segment has a pair of paddle-like structures called parapodia (almost feet). They
have a well-developed head. Mainly marine, like fanworms. FIG 33.24, pg. 655
3. Hirundinea (leeches)-feed on other inverts or are blood-sucking parasites. Once the leech makes
an incision into its host, it releases hirudin to keep the blood it is sucking from clotting. Body is
usually flattened with reduced coelom and segmentation. FIG 33.24, pg. 655
VI. Nematodes are nonsegmented pseudocoelomates covered by a tough cuticle
 Phylum Nematoda (Roundworms) FIG. 33.26, page 655.
 ≈ 90,000 species, ranging in size from 1 mm to more than a meter in length
 Found in most aquatic habitats (including the moist tissues of plants and animals)
 Covered by a cuticle, they do not have segments
 Have a complete digestive tract (alimentary canal) and lack a circulatory system
 Usually reproduce sexually with internal fertilization
 Play a key ecological role by decomposing and nutrient cycling
 Include many agricultural pests and parasites. They are very well-adapted to the parasitic life. FIG
33.27, page 656.
VII. Arthropods are segmented coelomates that have an exoskeleton and jointed appendages
 Phylum Arthropoda (crustaceans, spiders, and insects) FIG 33.29, page 657.
 Have regional segmentation, jointed appendages, and an exoskeleton (external skeleton)
 About a billion billion individuals, and almost 1 million species, mostly insects {two out of every
three organisms are arthropods}
 Very diverse and found everywhere on earth
 Some will shed exoskeleton in a process called molting, which is exhaustive to the Arthropod and
leaves it weakened. The exoskeleton is critical to the arthropods success on earth!
 Well developed sensory equipment, like eyes, olfactory receptors, and antennae
 Open circulatory system where fluid called hemolymph is propelled by a heart through short arteries
and then into spaces called sinuses surrounding the tissues and organs.
 A variety of organs for gas exchange, including gills and tracheal systems.
 Trilobites-all extinct today, very pronounced segmentation are great examples of early arthropods.
FIG 33.28, pg. 657.

Four subgroups:
1. Cheliceriforms (eurypterids {water scorpions}, horseshoe crabs, scorpions, spiders, mites and
ticks)-Named for their claw-like feeding appendages called chelicerae (pincers or fangs). This
group lacks antennae and has simple eyes. Groups include eurypterids and arachnids:

Arachnida (scorpions, spiders, ticks, and mites). Arachnids have cephalothorax with six
pairs of appendages: the chelicerae, the pedipolyps that function in feeding, and four
walking legs. FIG 33.32, pg. 659. In spiders, gas exchange is carried out by book lungs and
many build webs of silk, which is a protein made by glands and spun by spinnerets into
fibers. FIG. 33.31, pg. 659 for pictures
2. Myriapods (centipedes and millipedes). Terrestrial with jaw-like mandibles. They also have one
or two pairs of sensory antennae and a pair of compound eyes. Classes include:

Diplopoda=Millipedes: worm-like with two walking legs per segment. Eat decaying leaves
and other plant matter. FIG. 33.33, pg. 659.

Chilopoda=Centipedes: carnivores with jaw-like mandibles and one walking leg per
segment. FIG 33.34, pg. 660
3. Hexapods (insects and their wingless, six-legged relatives)

Insecta=Insects: most numerous class, live in most terrestrial habitats, freshwater, and they
fill the air. (NOTE: Entomology is the study of insects). Flight is one key to the great
success of insects. The major orders of insects are in Figure 33.37, pgs. 662-663
 Complete digestive system
 Open circulatory system
 Wastes are removed by Malpighian tubules
 Gas exchange occurs in a tracheal system
 Many undergo metamorphosis: Fig. 33.36, pg. 661
i.
Incomplete metamorphosis: (grasshoppers) young resemble adults but are
smaller and have different proportions
ii.
Complete metamorphosis: Have larval stages for eating and growing, like
the maggot, grub, or caterpillar. Theses stages are very different in
appearance from the adult.
 Sexual reproduction with internal fertilization
 Figure 33.35 on page 660 shows the anatomy of an insect
4. Crustaceans (crabs, lobsters, crayfish, and shrimp) FIG 33.38, pg. 664

≈ 40,000 species, mainly in marine and freshwater ecosystems

Specialized branched appendages, even on the abdomen. Usually these appendages can be
regenerated if they are broken

Gas exchange occurs by the cuticle or gills

Open circulatory system

Separate sexes

Larval stages

Three groups:
1. Isopods: small, marine and land species
2. Copepods: very numerous, marine and freshwater plankton
3. Decapods: Lobsters, crayfish, crabs, and shrimp. Relatively large with a cuticle of
Calcium Carbonate. Mainly marine but some are freshwater.
VIII. Echinoderms and chordates are deuterostomes
 Coelomates
 Phylum Echinodermata “spiky skin” FIG. 33.39, pg. 665
 Sessile or slow moving
 Parts radiate from center, often as 5 spokes
 A skin covers the hard calcareous plates
 They have a water vascular system, which is a network of hydraulic canals branching into extensions
called tube feet. The tube feet function in locomotion, feeding, and gas exchange.
 Separate male and females, usually release gametes in water
 ≈ 7,000 species in 6 classes (FIG 33.40, page 667):
1. Asteroidea (sea stars) have at least five arms that radiate from a central disk. The arms bear tube
feet that suction to rocks, aid in movement, or grasp and open pray. These arms can regenerate.
2. Ophiuroidea (brittle stars) have a distinct central disk and long arms. No suckers on their tube
feet, they move by lashing the arms. Suspension feeders, predators, or scavengers.
3. Echinoidea (sea urchins and sand dollars) have no arms, but they have five rows of tube feet from
movement. The mouth is a complex jaw-like structure used for eating seaweeds.
4. Crinoidea (sea lilies and feather stars) s.l. attach to substrate by stalks and f.s. crawl using arms.
Feed by suspension. An ancient class.
5. Holothuroidea (sea cucumbers) lack spines, and have a reduced endoskeleton. Have five rows of
tube feet, and an elongated oral axis.
6. Concentricycloidea (sea daisies) a new group that lives on waterlogged wood in the deep sea.
They have a disk-shaped body ringed with small spines.
TABLE 33.7, page 668 is a GREAT STUDY GUIDE