Invertebrate Study Guide
Key Terms and Study Questions
There are many types of invertebrates. They are the most numerous types of animals in the
world. The main groups are simple animals, animals with stinging cells, worms, mollusks,
echinoderms, and arthropods.
Animals will have either radial or bilateral symmetry. Radial symmetry is when the body of
the animal is in a round shape with no distinguishable left or right side (such as a sea star).
Typically, the mouth is in the center of the body with arms or tentacles around the center.
Bilateral symmetry is when an animal has two sides to their body – such as a right and left
side. Humans, dogs, cats, ants, fish, etc. all have bilateral symmetry.
Simple animals such as sponges are considered simple in structure and eat food that enters
into their pores.
Cnidarians are animals with stinging cells, such as sea jellies (also called jelly fish), sea
anemone, hydras, Portuguese man-of-war, are more complex in structure than sponges and
use tentacles to stun their food.
Worms are either flat, round, or segmented. Of the three, segmented worms are the most
complex. An earthworm is an example of a segmented worm. Some worms, such as planarian (a
flat worm), are able to regenerate (or grow back body parts that are missing).
Many types of worms are parasites. These creatures enter into a ‘host’ animal - usually as an
egg and begin to grow once inside the animal’s digestive system. These animals can make the
host animal sick as the parasite grows eating the digested food that the animals eats. Dogs and
cats will sometimes get parasites that can be controlled with medicines.
Mollusks include slugs, snails, octopi, squids, and clams. Mollusks are soft bodied animals.
Some mollusks, such as clams and snails have a hard outer covering (shell). The giant squid is
the largest of the mollusks and lives deep in the ocean and is rarely seen by humans. Mollusks
are classified in three ways:



Gastropod – a mollusk, usually with a shell and muscular foot (snails, slugs)
Bivalves – a mollusk with two shells that come together at a hinge (clams, oyster,
scallop)
Cephalopod – a mollusk with a head and tentacles (squids and octopi)
Echinoderms are spiny and rough skinned creatures. A sea star and sea cucumber are
examples of echinoderms. Sea stars eat clams and pull them apart using the hollow tube feet to
pry open the shell of their prey. Once a clam is opened, the sea stars use external digestion by
opening its stomach on the clam and covering the defenseless soft-bodied animal. The clam is
killed and the remains are eaten by the sea star.
The largest group of animals is the Arthropods. Arthropods include millipedes, centipedes,
crustaceans, arachnids, and insects. All arthropods have a segmented body, jointed legs, and an
exoskeleton.
Millipedes and centipedes are arthropods with many legs and segments. Each segment of a
centipede has two legs. Each segment of a millipede has four legs.
Crustaceans are arthropods with eight walking legs and two legs that form claws. Crustaceans
have antennae that they use to touch, smell, and taste. Lobsters, shrimp, and crabs are types of
crustaceans.
Arachnids include spiders, scorpions, ticks, and mites. Arachnids have eight legs and two body
segments - a cephalothorax (head and thorax in one segment) and an abdomen. Spiders all
have venom and make silk. Only a small number are harmful to humans. Spiders are helpful to
humans because they control the insect population.
Insects are the largest group of arthropods. Insects all have six legs and antennae. Some
insects have wings.
Some insects are harmful - such as termites (eat wood in homes), cockroaches (spread disease),
Flies (spread disease), mosquitoes (spread disease), and boll weevil (destroys crops).
Some insects are helpful - such as dragonflies (which kill mosquitoes and flies) and bees (which
make honey).
Some insects, such as bees, termites, and ants are social insects. These insects live as a
collective in a community where each insect has a different job. Jobs include workers, warriors,
scouts, and the queen. The success of the colony of these insect depends on all members doing
their job and the queen laying eggs.
An exoskeleton is a skeleton on the outside of the body. Exoskeletons do not grow. As an
animal grows, it sheds its exoskeleton and forms a new one. This process is called molting.
Metamorphosis is a dramatic change in appearance.
Insects undergo two forms of metamorphosis:
Incomplete metamorphosis (3 stages):
Egg
Nymph
Adult
Complete metamorphosis (4 stages):
Egg
Larva
Pupa (cocoon)
Adult
Questions:
1.
Compare and contrast a beetle and a spider.
2.
How can insects be both pests and helpful to humans
3.
What does a mosquito’s mouth parts look like?
4.
Why are centipedes and millipedes considered arthropods and not worms?
5.
Explain why we consider ants and bees ‘social insects’?
Word Bank:
Pedipalps
4 pairs of legs
antennae
3 pairs of legs
wings
mouth parts
Compound eye
thorax
Label the main parts of these arthropods:
cephalothorax
fangs
spinneret
head
abdomen