Coelenterates and Worms

Phylum	Layers	Body cavity	Representative
Coelenterates	2	gut	hydra
Flatworms	3	none	planarian
Roundworms	3	primary	Ascaris
Annelids	3	secondary	leech

Coelenterates

Hydra, jellyfish, sea anemones, coral polyps. Two-layered (ectoderm and endoderm), with radial symmetry.

They have a gastric (gut) cavity. The nervous system is net-like (diffuse). They have stinging cells for defense.

Worms

• Flatworms (white planarian, liver fluke, beef tapeworm): three-layered, bilaterally symmetrical, no body cavity.
• Roundworms (Ascaris, pinworm): have a primary body cavity and an anus.
• Annelids: earthworm, leech. They have a secondary body cavity and a closed circulatory system.

Key facts

• Hydra: high regeneration ability, reproduces by budding. Digestion takes place inside cells and in the gut cavity.
• The intermediate host of the liver fluke is the small pond snail, and the definitive host is cattle or sheep.
• The intermediate host of the beef tapeworm is cattle (Finnes), and the definitive host is humans.
• In roundworms, the anal opening appeared for the first time.
• In annelids, the circulatory system appeared for the first time.
• The earthworm loosens the soil and increases humus (it takes part in soil formation).