Animal Phyla: Mollusca, Arthropoda

Phylum Mollusca: The Molluscs

The phylum Mollusca is a diverse collection of animals that contains snails and slugs, bivalves, octopus, squid, and the chambered nautilus. They are found in all habitats. The word Mollus- means “soft” in Latin, which refers to the soft bodies, or visceral mass of body organs, that all molluscs share. Molluscs display bilateral symmetry.

Most members of the Phylum Mollusca share these characteristics:

  1. hard, calcium carbonate (limestone) shell that is secreted by the mantle (a protective layer of tissue surrounding organs)
  2. muscular foot used for motility or grasping
  3. gills for respiration
  4. coelom (with mouth and anus)
  5. radula , a rasping tool used for scraping algae or drilling into/catching prey

These common features have been modified in many different ways throughout the variety of Molluscs.

Watch the following movie clip from The Shape of Life to learn more about the MolluscaWhile viewing, record important ideas on your Fact Sheet.

A review of the Mollusca body systems:

  1. Mollusks use a muscular foot for movement, which can be modified into arms or tentacles in some species.
  2. Most mollusks breathe through gills. Some have lungs!
  3. All mollusks except bivalves contain a rasping, tongue-like radula for scraping food.
  4. The circulatory system consists of a three-chambered heart and open circulatory system (except for octopus & squids that have a closed circulatory system).
  5. Reproduction is sexual even in hermaphroditic forms. Mollusks go through a free swimming larval stage called the trochophore.
  6. A nervous system is present but is much more highly developed in cephalopods such as the octopus.

Examine the diagram below that shows the anatomy of a snail, a representative mollusc. What body system organs can you locate?


Members of this group are economically important as sources of human food, pearl and shell production, crop & flower damage, destruction to submerged wooden structures, and intermediate hosts for some parasitic diseases. The giant squid and giant clam are the two largest invertebrates.

Three groups of Molluscs are most well known to us. Study the table below for a comparison of each:


gastro = stomach

pod = foot


  • 80% of known Molluscs are Gastropods.
  • Most snails are marine, but some do live on land.
  • Marine snails have gills. Land snails have an air hole/lungs for breathing.
  • Open circulatory system.
  • Radula present.


Examples: snails, slugs


Land snail


bi = two

valvia = shell


  • These mollusks have two shells hinged together by a ligament.
  • Strong adductor muscles open and close the shells.
  • Current and excurrent siphons circulate water containing food and oxygen through the bivalve.
  • Gills extract oxygen from the water.
  • Muscular foot can be extended from the shell for movement or anchoring.
  • Open circulatory system.
  • No radula.
Examples: clams, oysters, mussels, and scallops


A scallop (the small dots are eyes!)


cephalo = head

pod = foot

  • Muscular foot is modified into tentacles or arms.
  • Except for the chambered nautilus, shell is greatly reduced to an internal break (octupus) or pen (squid).
  • Well-developed nervous system; the most intelligent of all invertebrates.
  • They use siphons to move by jet propulsion.
  • Gills for respiration.
  • Closed circulatory system.
  • Radula present.
  • An octupus can spit out an inky substance to help them escape from predators.
Examples: Octopus, squid, chambered nautilus


The chambered nautilus

Phylum Arthropoda

The Phylum Arthropoda is the largest and most diverse of all animal phyla. Arising first in the ocean, they were the first animals to invade land. The word “arthropod” means jointed foot and refers to their characteristic jointed appendages.

All arthropods have 3 distinct features that make them recognizable as arthropods:

  1. segmented body: head, thorax (or collectively the cephalothorax), and abdomen.
  2. jointed appendages such as legs, antenna, and mouthparts.
  3. a hard exoskeleton usually composed of substance called chitin; this requires arthropods to molt in order to grow.

Each of these body features has allowed the arthropods to adapt to many different environments and become incredibly diverse and successful.

Watch the following two movie clips from The Shape of Life to learn more about the aquatic and terrestrial arthropods. While viewing, record important ideas on your Fact Sheet.


A review of some general features of Arthropods:

  1. Body muscles attach to the inside of the exoskeleton.
  2. Open circulatory system (blood is pumped out of blood vessels into the body).
  3. Nervous system with an anterior brain and a variety of sensory organs: compound eyes, or simple eyes called ocelli, tympanic membranes for hearing, and antennae for smelling, feeling, and tasting.
  4. Malpighian tubules filter wastes in arthropods.
  5. Respiration varies depending on the arthropod’s habitat:terrestrial arthropods (insects, centipedes and millipedes) use trachea (hollow air tubes); aquatic chelicerates (horseshoe crab) have book gills; spiders, ticks, and scorpions use book lungs; and crustaceans breathe through gills.

The phylum Arthropoda is divided into subgroups according to each organism’s body segments and body parts (antennae, legs and wings). Study the differences among each of the Arthropod groups.


Metamorphosis is the process of transformation from an immature to adult body structure in multiple stages. Usually this body structure change is coupled with a change in habitat or behavior. Metamorphosis is commonly seen in the insects, but is also common in other animal phyla (such as amphibians or fish, in the Phylum Chordata).

There are two types of metamorphosis we see in insects:

  1. Incomplete metamorphosis – Insect progresses through egg, nymph, and adult stages. The nymph stage often looks very similar to the adult, but smaller. Examples: Dragonflies and grasshoppers.
  2. Complete metamorphosis – Insect progresses through egg, larvae, pupa and adult stages. Example: butterflies, bees, flies, beetles.

Insects such as silverfish and fleas do not go through metamorphosis. Hormones control metamorphosis and molting.