Arthropods and echinoderms are invertebrate animals that share some key characteristics. Arthropods have segmented bodies, jointed limbs, an external skeleton, and a well-developed nervous system. They include insects, spiders, crustaceans, and others. Echinoderms have a spiny endoskeleton and radial symmetry as adults, though their larvae have bilateral symmetry. They use a water vascular system and include sea stars, sea urchins, sea cucumbers, and others. Both groups display diverse adaptations for movement, feeding, and survival.

1. Arthropods & Echinoderms

2. • Characteristics of Arthropods: all share 4
characteristics
(1) A segmented body with specialized parts
-segments include specialized parts, such as
wings, antennae, gills, pincers, and claws
-3 main body parts: head, thorax, and
abdomen

3. (2) Jointed Limbs
-gives arthropods their name: “arthro” for
joint, and “pod” for foot
-jointed limbs makes it easier to move
(3) External Skeleton
-made of protein and chitin
-supports body, allows movement
-acts like a suit of armor to protect organs
-also keeps water inside animals’ body so
it can live on land without drying out

4. (4) Well Developed
Nervous System
-receives
information from
sense organs,
including eyes and
bristles
-most arthropods
have compound
eyes, which is made
of many identical,
light-sensitive units

5. • Arthropods are classified by the kinds of
body parts that they have
• You can tell differences between them
by the number of legs, eyes, and
antennae that they have

6. (1) Centipedes and Millipedes
• One pair of antennae, a hard head,
and one pair of mandibles (mouthparts
that can pierce and chew food)
• Count the number of legs to tell the
difference between them!
centipede millipede

7. (2) Crustaceans
– Shrimps, barnacles,
crabs, and lobsters
– Live in water
– Have gills for
breathing, mandibles
for eating, and two
compound eyes on
the ends of
eyestalks
– Unlike all other
arthropods,
crustaceans have
two pairs of
antennae

8. (3) Arachnids
– Spiders, scorpions,
mites, and ticks
– Two main body parts:
cephalothorax and
abdomen
– Most have four pairs of
legs and have no
antennae
– Instead of mandibles,
they have pair of
clawlike mouthparts
called chelicerae
– Simple eyes

9. – Few spider bites
need medical
treatment, but
chelicerae of many
spiders cannot
even pierce human
skin
– Spiders kill more
insect pests than
any other animal
– Ticks are parasites
that live off of
hosts’ blood

10. (4) Insects
– Make up the
largest group of
arthropods
– All have three main
body parts, six
legs, and two
antennae
– Also have two
compound eyes
and mandibles

11. – Metamorphosis: a phase in the life
cycle of many animals during which a
rapid change from the immature form of
an organism to the adult form takes
place
• Complete: four stages (egg, larva, pupa,
and adult), such as a butterfly
• Incomplete: three main stages (egg,
nymph, and adult), such as grasshoppers
and cockroaches
– Nymph may shed exoskeleton several
times (called molting)

13. Echinoderms
• Echinoderms: spiny
invertebrates, such as
starfish, sea urchins, and
sand dollars
• Endoskeleton: an
echinoderm’s internal
skeleton, which covers the
animal’s skin
• Adult echinoderms have
radial symmetry, but they
develop from larvae that
have bilateral symmetry

14. *Nervous System
– Simple nervous system similar to that of a jellyfish
– Around the mouth, they have a circle of ring
fibers called “the nerve ring”
– In sea stars, radial nerve runs from nerve ring
to tip of each arm and controls movement
*Water Vascular System
– System of canals filled with fluid
– Uses water pumps to help animal move, eat,
breathe, and sense its environment

16. Kinds of Echinoderms
• (1) Brittle Stars and Basket Stars
– Long, slim arms and often smaller than sea
stars
– Don’t have suckers on tube feet

17. • (2) Sea Urchins and Sand
Dollars
– Round and
endoskeleton forms a
solid, shell-like
structure
– No arms; use tube feet
for movement
– Sea urchins feed on
algae and sand dollars
just bury in sand and
eat tiny particles of food

18. • (3) Sea Lilies and
Feather Stars
– May have 5 to 200
feathery arms
– Arms stretch away
from body and trap
small pieces of
food
– Sea lilies sit on top
of a stalk; feather
stars do not have a
stalk

19. • (4) Sea Cucumbers
– Has a soft,
leathery body and
no arms
– Unlike other
echinoderms, sea
cucumbers are
long and have a
wormlike shape
– Move using tube
feet

20. Is that a Fact?!?!?!
• They have the peculiar adaptation of
expelling first sticky threads, perhaps to
incapacitate predators, and then their
internal organs when startled by a
potential predator. These organs can then
be regrown.

21. • (5) Sea Stars
(starfish)
– Most familiar
echinoderm