Chapter 15.2Mollusks and Annelid Worms

Mollusks Snails, slugs, clams, oysters, squids

and octopuses are all mollusks Most mollusks fit into 3 classes

Gastropods which include slugs and snails

Bivalves which include clams and other shellfish that have 2 shells

Cephalopods include squids and octopuses

How do mollusks eat? Each mollusk has its own way of

eating. Snails and slugs eat with a ribbon like

organ – a tongue covered with curved teeth. This organ is called a radula.

Clams and oysters attach to one place and use gills to filter tiny plants, bacteria and other particles from the water.

Octopuses and squids use tentacles to grab their food and to place it in their powerful jaws.

Ganglia and Brains All mollusks have complex ganglia.

They have ganglia to control breathing, movement and digestion.

Pumping Blood Unlike simple invertebrates mollusks

have a circulatory system. Open-circulatory system: a simple

heart pumps blood through blood vessels that empty into sinuses or spaces in the animals body.

Closed circulatory system: a heart pumps blood through a network of blood vessels that form a closed loop.

Mollusk bodies While mollusks may look very

different from each other, their inner structures are actually very similar. Foot : helps the animal move Visceral mass: the gills, the guts, and

other organs Mantle: a layer of tissue that covers

and protects the visceral mass Shell: the outside of the mantle.

Protects the mollusk from predators

Annelid Worms Often called segmented worms

because their bodies have segments A segment is an identical or almost

identical repeating body part. Like roundworms and flatworms

annelid worms have bilateral symmetry.

Annelid worms are more complex than other worms They have a closed circulatory system They have a complex nervous system

with a brain A nerve cord connects each segment to a

ganglion in the brain

Three Major Groups of Annelid Worms Earthworms: Earthworms are

the most familiar Each earthworm has 100 to 175

segments Each segment has a special job

Earthworms eat material in the soil. They break down plant and animal matter in the soil and leave behind wastes called castings. They also improve garden soil by

creating tunnels that allow air and water to reach deep into the soil

To move earthworms use stiff hairs or bristles to move through the dirt

Marine Worms These worms are called polychates

(which means many bristles) They also come in many bright colors

Most live in the ocean

Leeches Known as parasites that

suck other animals blood. Some are not parasites,

these eat dead animals They can be useful to

help restore blood flow to a limb and to also prevent blood clots.