Earthworms are the most common segmented worms.

Flatworms have a simple brain called GANGLIA, which is a bundle of nerves.

Roundworms (nematodes) are "threadlike worms." They have tube-like bodies that are pointed at both ends.

Animals that are not parasitic are called FREE-LIVING animals.

Parasitic animals live in the body of another animal for food.

Tapeworms cling to the intestinal wall of another animal using HOOKS and SUCKERS.

Tapeworms are flatworms that are found in the intestinal wall of another animal.

The brain of a flatworm is called a GANGLIA.

GILLS are special organs that help animals absorb oxygen from the water.

The three types of mollusks are UNIVALVES, BIVALVES, and CEPHALOPODS.

UNIVALVES have one shell. BIVALVES are hinged together. Squid and octopus are examples of CEPHALOPODS. The squid have highly reduced internal shell.

Worms are invertebrates with long, soft, usually cylindrical bodies, and without legs

Flatworms (Platyhelminthes) are the simplest types of worms.