Classification of Animals:
Invertebrate Animals
6th Grade Science
Animal Characteristics
Many-celled organisms sharing similar
features and that are made of different
kinds of cells.
Animal cells have a nucleus and organelles
surrounded by a membrane – EUKARYOTIC.
Cannot make their own food –
HETEROTROPHIC – digest their food.
Can move from place to place to find food,
shelter, and mates, and to escape from
Symmetry: arrangement of the
individual parts of an object
Radial: body parts arranged in a circle
around a central point
Bilateral: parts are mirror images of
each other
Asymmetrical: bodies cannot be divided
into matching halves
Animal Classification
Invertebrates ( No
Animal Kingdom
Cnidarians Roundworms
Vertebrates (Backbone)
Flatworms Mollusks
What is an Invertebrate?
Invertebrates are animals that do not have
97% of the animal kingdom is made up of
Some can be found in ponds, oceans, and other
water environments.
Insects and some other invertebrates have
An exoskeleton is a hard outer covering that
protects an animal’s body and gives it shape.
Porifera: Sponges
Porifera Characteristics
They live in water. (Most are found in the
They look like plants but they are animals.
Sponges stay fixed in one place - SESSILE.
Their bodies are full of pores and their skeleton
made of spiky fibers (spicules) or rubbery
Sponges are divided into classes according to
type of spicule they have – 5,000 species
Porifera Characteristics
Sponges can reproduce asexually through
budding ~ GEMMULES; a new sponge grows
from pieces of an old sponge
Most sponges that reproduce sexually are
hermaphrodites, meaning they have both eggs
and sperm
Sperm is released into water
Sperm floats until they are drawn into another
sponge where they fertilize an egg
Larva develops in sponge, leaves sponge, and
settles to the bottom where it grows into an adult
Cnidaria: Corals, Hydras, and Jellyfish
Cnidaria Characteristics
Cnidaria comes for the Greek word for nettle.
All cnidarians have stinging cells called
NEMATOCYSTS in tentacles surrounding their
Cnidarians are more complex than sponges.
They have complex tissues, a gut for digesting
food, and a nervous system.
They come in two body shapes, the medusa
and the polyp.
Polyp: usually sessile and vase-shaped
Medusa: free-swimming and bell-shaped
Cnidaria Characteristics
Cnidarians reproduce both sexually
and asexually
Polyp forms reproduce asexually by
Some polyps also reproduce sexually be
releasing sperm or eggs
Medusa forms have a two-stage life
cycle in which they reproduce both
sexually and asexually
Sea Anenomes and Corals
They are polyps their entire life.
They look like brightly colored flowers.
They live in colonies.
They have soft tube-like bodies with a single
opening surrounded by arm-like parts called
They feed by catching tiny animals in their tentacles.
They live in fresh water.
They spend their entire
life as polyps.
Hydras have tentacles
that catch their food.
They move from place to
Hydras are very small animals.
Reproduce asexually by budding.
They spend most of
their life as medusa.
They swim.
Jellyfish catch
shrimp, fish, and
other animals in its
tentacles also.
Reproduces sexually to produce polyps; then
each polyp reproduces asexually to form new
Segmented Worms
Platyhelminthes: Flatworms
Search for their food; long, flattened bodies
with organs and systems
They have a head and a tail, and flattened
bodies – BILATERAL
Planaria – free-living
Tapeworms – parasitic; each segment
(proglottid) contains sperm and eggs
(reproduce sexually) ~ when fertilized eggs fill
segment, it breaks off and passes out with
wastes of host – can be up to 80,000 eggs
per segment!!!
Lack a digestive system and absorb nutrients from
Platyhelminthes: Flatworms
Nematoda: Roundworms
They have rounded bodies; body is a tube within a
tube; I.e., digestive tract has both a mouth and an
They live in damp places and they can
also live inside humans and other animals.
Heartworm (Dirofilaria immitis) ~
Passed by mosquito bite
They too can make people and other animals sick.
Diets vary with some roundworms being
decomposeers, some predators, and some
Annelida: Segmented Worms
The earthworm (oligochaete), leech (hirudinea), and
marine worm (polychaete) belong to this group.
Their bodies are divided into repeating segments
Each segment has nerve cells, blood vessels, part of the
digestive tract, and the coelom (body cavity)
Closed circulatory system and complete digestive system
with two body openings
They prefer burrowing through moist soil.
This allows them to move easily and it keeps them from
drying out.
Annelida: Segmented Worms
Earthworms – have more than 100
body segments
Use external bristle-like setae and
muscles to move
Eat organic material in soil
Exchange carbon dioxide and oxygen
through mucus-covered skin
Annelida: Segmented Worms
Have flat bodies with sucking disks at
both ends
Can store enormous amounts of food for
Secrete heparin,
which prevents
blood from clotting
Annelida: Segmented Worms
Marine worms – use bristles or setae
for moving
Some are filter feeders
Some eat plants or rotting material
Some are predators or parasites
Mollusca: Octopi, Squid, Slugs,
Snails, and Bivalves
A mollusk has a soft body usually covered by
a hard shell, a rough tongue (radula), a
muscular foot, and a mantle (thin layer of
tissue that covers the mollusk’s soft body and
secretes the shell).
Aquatic mollusks have gills for gas
exchange; land mollusks have lungs
A snail is a mollusk with a single hard shell.
A clam has two shells joined together by a hinge.
Squids and octopi are also mollusks.
Their hard shells are small, but they are inside
Characteristics of Mollusks
Mantle: tissue that covers a mollusk’s soft
body and that may produce a shell
Lungs or gills: exchange carbon dioxide
from the animal for oxygen in the air or
Many mollusks use a radula, a scratchy
tongue-like organ, to help them eat
Some have an open circulatory system
which washed blood over organs and lacks
blood vessels
Types of Mollusks
Gastropods – most have one shell
Live in water or on land
Move by gliding their large muscular foot
along a trail of mucus
Gastropods: Slugs and Snails
Types of Mollusks
Bivalves – have two shells
Large muscles open and close shell halves
Water animals that filter feed
Use gills to remove foot from
Bivalves: Clams and other twoshelled shellfish
Types of Mollusks
Cephalopods – have no external shell
Have a foot divided into tentacles with
Move by using a mantle to quickly
squeeze water through a funnel-like
Have a closed circulatory system with
blood vessels
Cephalopods: Octopi and Squid
Arthropoda: Insects, Spiders,
Ticks, Mites, Centipedes,
Millipedes, Crustaceans
Arthropoda Characteristics
Arthropods are a group of invertebrates with
jointed appendages, such as claws, legs, and
antennae, and a hard exoskeleton that
protects the arthropod; also have bilateral
More than a million species of arthropods
have been discovered!!
As it grows, it molts, or sheds its old
Then it grows a new exoskeleton that allows
its body to continue to grow.
Insect Characteristics
Insects have adapted to living almost
everywhere! Over 700,000 species have been
classified….so far!!
An insect’s body has 3 parts: the head, thorax,
and abdomen.
The head has one pair of antennae and two
compound eyes ~ well- developed sense organs.
Thorax has three pairs of jointed legs and usually
one or two pairs of wings.
Reproductive organs are located in abdomen.
Open circulatory system; oxygen enters through
Insect Metamorphosis
Complete Metamorphosis
Insect Metamorphosis
Incomplete Metamorphosis
Arachnids: Spiders, Scorpions,
Ticks, and Mites
They have 2 main
body parts: a cephalothorax
and an abdomen
The thorax has
4 pairs of jointed legs; no antennae.
They do have special
mouth parts like fangs.
They kill more insect
pests than any other animal.
Myriapods: Centipedes and
Centipedes use their
many legs to run
from enemies (one pair of
jointed legs attached to each segment).
Millipedes roll up
their bodies when
they sense danger approaching (two pairs of
jointed legs attached to each segment).
Feed on plants.
Crustaceans: Shrimp, Barnacles,
Crab, Crayfish, and Lobster
Almost all crustaceans are
aquatic & have gills.
All have 2 pairs of
antennae, three types of
chewing appendages, and
five pairs of legs.
Echinodermata: Starfish and Sea Urchins
Belongs to a group of invertebrates that
have tiny tube feet and body parts
arranged around a central area.
A starfish has five arms and no head!
The hard, spiny covering of the starfish
gives the animal protection.
A sea urchin belongs to this same
Its body is covered with spines.
Radial symmetry
Diets vary ~ predators, filter feeders,
some eat rotting material
Spiny skin covering an internal skeleton of
Water-vascular system to help them move
and eat
Some can reproduce through regeneration
from parts.
Sea Urchins

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