Mollusks,
Arthropods,
Echinoderms
Mollusks, Arthropods, Echinoderms
Mollusks
• Soft-bodied invertebrates
• Have bilateral symmetry
• Usually have one or two shells with
organs in a fluid filled cavity
• Most live in water
• Many different species
Mollusks’ Body Plan
• Mantle
– Thin layer of tissue that covers the body organs
• Mantle cavity (between soft body and mantle)
– Contains the gills that are used to breathe by
exchanging oxygen and carbon dioxide in the
water
• Open Circulatory System
– Most mollusk have this
– Moves blood through vessels and into open
spaces around body organs
Mollusks’ Body Plan
• Have a well developed head with a
mouth and some sensory organs
• Underside is a muscular foot
– Moves by making rhythmic contractions
Classification of Mollusks
• Classified into three common groups
based on shell presence, type and foot
type
– Gastropods
– Bivalves
– Cephalopods
Gastropods
• Largest group of mollusks
• Usually have a single shell
• Use a radula (a tongue-like organ with
rows of teeth) to get food
• Have foot glands that secrete a layer of
mucus for sliding
• Includes snails, conchs, and garden
slugs
The Radula
Obtaining Food
• Some gastropods are herbivores. These
include animals that eat only plants.
• Other gastropods are carnivores. These
include animals that eat only other animals.
Bivalves
• Have a hinged, two-part shell
• To open or close their shell they either
contract or relax their muscles
• Includes clams, oysters, scallops, & mussels
• Well adapted for water
– Clams can burrow in sand
– Mussels attach themselves to a solid surface
– Scallops escape predators by rapidly opening and
closing their shell
Pearls
Cephalopods
• Most specialized and complex
mollusks.
• Include squid, octopuses, and
chambered nautiluses.
• Have a well developed head
and many tentacles for
capturing prey.
• Closed circulatory system
– Moves blood through the body in
a series of closed vessels like
humans.
• Use jet propulsion to move at
speeds of 6 m/s.
Origin of Mollusks
• Mollusk fossils date to more than 500
million years ago
• Some species of mollusk, like the
chambered nautilus, have changed
very little from their ancestors
• Today’s mollusks are descendants of
ancient mollusks
Value of Mollusks
• Provide food for people and other animals
• Many people make their living raising or
collecting mollusks to sell
• Shells can be used for jewelry and decoration
• Pearls are produced by several species of
mollusks most are made by pearl oysters
Negative Effects of Mollusks
• Land slugs and snails can damage plants
• Certain species of snails are hosts for
parasites that can infect humans
• Bacteria, viruses, and toxic protists can
become trapped in these animals because
they are filter feeders—eating them could
result in sickness or even death
Arthropods
• Characteristics
– Largest group of animals
– Have jointed appendages which include
legs, antennae, claws and pincers
– Have bilateral symmetry, segmented
bodies, exoskeletons, a body cavity, a
digestive system with two openings and a
nervous system
– Most have separate sexes and reproduce
sexually
Origin of Arthropods
• Some fossils are more than 500 million years
old
• Scientist hypothesized that arthropods
probably evolved from an ancestor of
segmented worms because they have body
segments
• The hard exoskeleton and walking legs
allowed arthropods to be among the first
animals to live successfully on land
Value of Arthropods
• A source of food
• Agriculture would be impossible without
bees and other insects to pollinate
crops
• Useful chemicals are obtain from some
arthropods
• Important part of ecological community
Arthropods
• Body Segments
– Bodies of these animals are divided into segments
similar to segmented worms
– Some have many segments, others have
segments that are fused together to form body
regions
• Exoskeleton
– A hard outer covering that supports and protects
the internal body and provides places for muscle
to attach.
– Doesn’t grow as the animals does, it is shed and
replaced during a process called molting
Crustaceans
• Have one or two pair of antennae and
mandibles, which are used for crushing
food.
• Most live in water, but some live in
moist environments on land—such as
pill bug.
• Have five pair of legs, first pair of legs
are claws for catching and holding food.
Crustaceans
• Swimmerets are appendages on the
abdomen which help in movement and
are used in reproduction; also force
water over the gills used in O2 and CO2
exchange
• If a crustacean loses an appendage it
can regenerate it
Arachnids
• Scorpions
– Have sharp, poison filled stinger at the end of
abdomen.
– Have a well-developed appendages which they
can grab their prey.
• Spiders
– Can’t chew their food, release enzymes into prey
to digest it—then suck the predigest liquid into its
mouth.
– Have book lungs where O2 and CO2 are
exchanged.
Arachnids
• Mites & Ticks
– Most are parasites
– Ticks have specialized mouthparts to
remove blood from the host.
– Ticks often carry disease such as Lyme
disease.
Centipedes & Millipedes
• Have long bodies and many segments,
exoskeleton, jointed legs, antennae and
simple eyes.
• Found in damp environments
• Reproduce sexually
• Make nests for eggs and stay with them until
they hatch.
• Centipedes are predators
• Millipedes feed on decaying plant matter.
Insects
• Have three body regions
• Head
– Has a pair of antennae, eyes and a mouth
• Thorax
– Three pairs of legs and one or two pairs of
wings if present are attached here
• Abdomen
– Where reproductive structures are found
Insects
• Have an open circulatory system that carries
digestive food to cells and removes wastes
• Insect blood doesn’t carry O2 instead air
enters and exits through openings called
spiracles found on the abdomen and thorax
• Are the only invertebrate animals that can fly
Insects & Food
• Feed on a number of things have different
mouth parts to obtain food
• Grasshoppers and ants have large mandibles
for chewing
• Butterflies and honey bees have siphons for
lapping up nectar
• Aphids and mosquitoes have mouth parts
that are adapted for piercing into plants or
other organisms
Metamorphosis
• A series of changes that an insect goes through
• Two types
– Complete
• Includes stages of egg, larva, pupa, and adult
• Ex. Butterflies, bees, flies
– Incomplete
• Includes stages of egg, nymph, adult
• The nymph form molts several times before becoming an
adult
• Ex. Grasshoppers, crickets
Insects success
• Insects are extremely successful based
these reasons
– Tough flexible, waterproof exoskeleton
– Ability to fly
– Rapid reproduction cycles
– Small sizes
• Insects have other adaptations that
allow them to be successful
Controlling Insects
• Not all arthropods are of value some
are pests that carry disease or can
damage crops
Controlling Insects
• Common ways to control insects
– Insecticides, but these also kill non-harmful
insects
– Biological controls
• Types of bacteria, fungi, and viruses can be
used to control insects
• Natural predators being released to kill the
harmful insect
• Some how interfere with reproduction of the
particular insect
Echinoderms
• Characteristics
– Have an endoskeleton covered by a thin, bumpy
or spiny epidermis
– Radial symmetrical—allowing them to sense food,
predators and other things in the environment
from all directions
– Have mouth, stomach, intestines
– Feed on a variety of plants and animals
– Have no head or brain, but have a nerve ring that
surrounds the mouth
– Also have cells that respond to light and touch
Water-Vascular System
• A characteristic unique to echinoderms
• Allows them to move, exchange CO2 and O2,
capture food, and release wastes
• It is a network of water-filled canals with
thousands of tube feet connected to it.
• Tube feet—hollow, thin walled tubes that
ends in a suction cup.
– As pressure in the tube feet changes the animal is
able to move along by pushing out and pulling in
its tube feet
Characteristics of Echinoderms
• Echinoderms, such as this sea star, have a water
vascular system that helps them move and catch
food.
Types of Echinoderms
• Sea Stars
– Echinoderms with at least 5 arms arranged
around a central point
– Uses tube feet to open shells of prey, once
open pushes its stomach into shell and
uses an enzyme to digest it
– Reproduce sexually
– Can repair themselves by regeneration
Types of Echinoderms
• Brittle Stars
– Have fragile, slender, branched arms that
break off easily
– This adaptation allows them to survive
– They can regenerate broken off body parts
– Use flexible arms for movement and tube
feet to get food into their mouths
Types of Echinoderms
• Sea Urchins & Sand Dollars
– Disk or globe-shaped animals covered in
spines
– Spines help in movement and in burrowing
– Also can protect them from predators
– Sea Urchins have five tooth like structures
around their mouth
Types of Echinoderms
• Sea Cucumber
– Soft bodied echinoderms
– Have a leathery covering
– Have tentacles around their mouth and
rows of tube feet on their upper and lower
surfaces
– When threatened, they may expel their
internal organs which will then be
regenerated in a few weeks
Value of Echinoderms
• Feed on dead organisms in the marine
environment
• Help recycle material
• Used for food
• Possible sources of medicine
• Sea stars can help control the
population of other organisms
Origin of Echinoderms
• Date back more than 400 million years ago
• Earliest echinoderms had bilateral symmetry
as adults
• Scientists hypothesize that echinoderms more
closely resemble animals with backbones than
any other group of invertebrates
– Have similar embryos that develop similar to
vertebrates
– Complex body systems