Download Invertebrates Phylum Mollusca (Snails, Bivalves, Octopus, Squid)

Biology 20: Diversity of Life
Invertebrates – Teacher Notes
Invertebrates
Phylum Mollusca (Snails, Bivalves, Octopus, Squid)
 The word “Mollusca” means soft
 Bilateral symmetry
 Soft body with internal or external shell
 Have a muscular foot for movement - can
be modified into arms or tentacles
 Breathe through gills or lungs located below a protective layer called the
mantle
 The mantle forms the shell in some species and also protects the body
organs
 Visceral mass protects the internal organs
 All mollusks except bivalves contain a rasping, tongue-like radula for
scraping food.
 The circulatory system consists of a three-chambered heart and openflowing system except for octopus & squids which have a closed circulatory
system.
 Reproduction is sexual even in hermaphroditic forms. Mollusks go through
a free swimming larval stage called the trochophore.
 Classified according to their foot and shell. Three major classes of mollusks:
1. Gastropods (snails, slugs)
 shell-less or single shelled
 move by a muscular foot attached to their belly
 snails without shells are called slugs
Biology 20: Diversity of Life
Invertebrates – Teacher Notes
2. Bivalves (clams, oysters)
 have two shells hinged together by a ligament
 muscles open and close the shells
 siphons circulate water containing food and oxygen through
the bivalve
 gills extract the oxygen from the water
 move by jet propulsion
 muscular foot can be extended from the shell for movement or
anchoring.
3. Cephalopods (octopi, squid)
 soft-bodied , head is attached to foot
 foot is divided into tentacles with sucking disks
 most have beaks, tentacles and jaws and are active predators
 small internal shells or no shell at all
 the most intelligent of all invertebrates
 use siphons to move by jet propulsion.
Representative Mollusks:
a) Snails
o Gastropod meaning it walks on its belly
o Most snails are marine and have gills to breathe
o Land snails have an air hole for breathing
o Snails can be very large. The helmet snail can be as big as 15 pounds!
b) Squid
o Marine animals
o Blast clouds of ink when threatened
o Translucent body, can be iridescent, and can
change colors
o Are the fastest swimmers among the
invertebrates – some “flying squid” have been
known to leap out of the water at speeds of 20
mph
o The giant squid is the largest cephalopod. It can
be up to 60 meters in length and has been known
to eat whales.
Biology 20: Diversity of Life
Invertebrates – Teacher Notes
Phylum Arthropoda (Insects, Spiders, Crustaceans)
 The word “Arthropoda” means jointed
 Hard exoskeleton which is usually composed
of substance called chitin
 Go through periodic ecdysis (molting) as
they shed or molt their exoskeleton
 Have specialized body segments (head, thorax, cephalothorax, &
abdomen)
 Have jointed appendages such as legs, antenna, and mouthparts
 Have an open circulatory system (blood is pumped out of blood vessels
into the body)
 Further classified according to their type of appendages
Major Group Characteristics
Centipedes
Have flat bodies with segments. Each body segment
has one pair of legs. Has 1 pair of antennae.
Millipedes
Have round bodies with many segments. Each body
segment, except the first four, has two pairs of legs.
Has 1 pair of antennae.
Crustaceans Have two main body parts, two pairs of antennae,
large claws, and four pairs of legs. Most live in
water. Have gills for gas exchange.
Arachnids
Have two body sections and four pairs of legs. Have
book lungs for gas exchange.
Insects
Have three body sections and three pairs of legs.
Some have wings.
Examples
Centipedes
Millipedes
Lobsters,
shrimp,
crabs, and
crayfish
Spiders,
scorpions,
ticks
Ants, lady
bugs
Biology 20: Diversity of Life
Invertebrates – Teacher Notes
Arthropods make up the largest phylum in the animal kingdom.
Why have they been so successful?
a) First true walkers
b) Have adapted for all environments – land, water, and air
c) Exoskeleton provides protection
d) Exhibit social behavior (e.g. colonies of ants and their related hierarchy,
communication between bees)
Arthropods have both a helpful and harmful relationship with humans.
Helpful
 Pollinate plants
 Act as decomposers
 Eat things harmful to humans
 Produce materials (e.g. honey, silk)
Harmful
 Damage crops
 Vectors of diseases
 Compete with humans for
food and shelter
Phylum Echinodermata (Starfish, sand dollars, and sea cucumbers)
 The word "echinoderm" means spiny skin
 Radial symmetry with five arms that extend
from a middle body section
 Have an endoskeleton made up of spines
 Adults have no head or brain and move by
extendable tube feet.
 Have a water vascular system made up of a system of canals that help the
organism feed and move. This hydraulic water system is strong enough to
help starfish open clam shells.
Biology 20: Diversity of Life
Invertebrates – Teacher Notes

Skin gills are used for respiration and waste removal.
 Can reproduce asexually by regeneration whenever parts are dropped or by
fragmentation
 Can reproduce sexually with external fertilization
Representative Echinoderm:
a) Starfish
o Have 5 arms set off from a central disk and their mouth located on
the underside or oral surface
o Are active marine predators
o Consume clams by turning their stomach inside out and sticking it
into the clam shell to digest the clam