The Good, The Bad, and the Spineless

Invertebrate Characteristics

• They are all very different from each other

• Examples: jellyfish, clams, earthworms, grasshoppers

• Have 3 different basic body plans, or types of symmetry

• Can be bilateral, radial, or asymmetry (no symmetry at all)

Asymmetrical: cannot draw a straight line to

divide its body into two or more equal parts. Its body is not organized

around a center.

Bilateral: the two sides of its body mirror each other.

Radial: Its body is

organized around the center, like

spokes on a wheel

Neurons and Ganglia

• All animals except sponges have special tissues that make fibers called neurons, which allow animals to sense their environment

• Neurons also carry messages around the body to control animal’s actions

• Simple invertebrates have neurons arranged into a nerve cord

• In some, many nerve cells come together as ganglia

• Each ganglion controls different parts of the body

•Ganglia are controlled by the brain

• Gut: pouch lined with cells that release chemicals that break down food into small particles

• In complex animals, gut is inside a coelom

• Coelom: body cavity that surrounds the gut and contains many organs, such as the heart and lungs (but kept separate)

• Keeps gut movement from disturbing other body processes

• Sponges cannot move and do not have gut

• A sponge sweeps water into its body through its pores

• Water flows into cavity in the middle of the body, bringing oxygen and food

• Special cells called collar cells line the cavity and filter and digest food

• Water leaves the body through a hole in the top of the sponge called the osculum

• Sponges have unusual abilities—if you force it through a strainer, the parts come back together and reform a sponge—called regeneration.

• A sponge’s skeleton supports its body and helps protect it from predators

• Most sponges have skeleton made of small, hard fibers called spicules– Some are straight, curved,

or have complex star shapes

• Sponges are divided into groups based on kind of skeleton it has

• Cnidarians– Invertebrates with stinging cells– Just like sponges, if the body cells are separated,

they can come back together to re-form the cnidarian– Two body forms

• Medusa: swim through water• Polyps: usually attach to surface

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• All cnidarians have tentacles covered with stinging cells

• When an organism brushes against the tentacles, it activates hundreds of stinging cells

• Each stinging cell uses water pressure to fire a tiny, barbed spear into the organism

• The tiny spears can release a painful—and sometimes paralyzing—poison into their targets

• Cnidarians use their stinging cells to protect themselves and to catch food

1. Hydrozoans: spend entire life as polyp

2. Jellyfish: spend of life as medusas, and catch food in tentacles

3. Sea anemones and corals: 1. Spend their lives as

polyps2. Often brightly colored

Flatworms• Simplest worms are

flatworms• All flatworms have bilateral

symmetry• Many have a clearly defined

head and two large eyespots (can sense direction of light)

• Some have bumps on side of head called “sensory lobes”, used for detecting food

• Three major classes:– Planarians and marine

flatworms– Flukes– Tapeworms

• Planarians– Life in freshwater lakes and streams– Most are predators– Its head, eyespots, and sensory lobes are

clues that it has a well-developed nervous system

• Flukes– Parasites– Most live and

reproduce into bodies of other animals

– Flukes have tiny heads without eyespots or sensory lobes

– Have special suckers and hooks for attaching to animals

• Tapeworms– Small head with no

eyespots or sensory lobes

– Live and reproduce in other animals

– Feed on these animals as parasites

– Tapeworms do not have a gut—they just attach to intestines of another animal and absorb nutrients

Roundworms• Have long, slim, round bodies

(bilateral symmetry) and a simple nervous system

• Most species are very small: a single rotten apple could contain 100,000 roundworms!

• Some break down dead material, others are parasites

• One roundworm causes the disease trichinosis