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Biology 20: Diversity of LifeInvertebrates – 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 open-flowing 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 LifeInvertebrates – 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) Snailso Gastropod meaning it walks on its bellyo Most snails are marine and have gills to breatheo Land snails have an air hole for breathingo Snails can be very large. The helmet snail can be as big as 15 pounds!
b) Squido Marine animals o Blast clouds of ink when threatenedo Translucent body, can be iridescent, and can
change colorso 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 LifeInvertebrates – 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 ExamplesCentipedes Have flat bodies with segments. Each body segment
has one pair of legs. Has 1 pair of antennae.Centipedes
Millipedes Have round bodies with many segments. Each body segment, except the first four, has two pairs of legs. Has 1 pair of antennae.
Millipedes
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.
Lobsters, shrimp, crabs, and crayfish
Arachnids Have two body sections and four pairs of legs. Have book lungs for gas exchange.
Spiders, scorpions, ticks
Insects Have three body sections and three pairs of legs. Some have wings.
Ants, lady bugs
Biology 20: Diversity of LifeInvertebrates – 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)
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.
Harmful Damage crops Vectors of diseases Compete with humans for
food and shelter
Biology 20: Diversity of LifeInvertebrates – 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) Starfisho Have 5 arms set off from a central disk and their mouth located on
the underside or oral surfaceo Are active marine predatorso Consume clams by turning their stomach inside out and sticking it
into the clam shell to digest the clam