Sponges are the simplest multicellular animals, with two cell layers and a jelly-like substance in between. They reproduce asexually by splitting into pieces. Cnidarians like jellyfish have radially symmetrical bodies with tentacles around their mouth. They also include corals. Worms include flatworms, roundworms, and segmented worms like earthworms. Spiny-skinned animals like starfish and sea urchins have thin skins covered in spines and tube feet. Mollusks mostly live in salt water and many have shells, while some like squids and octopuses do not. Arthropods are another invertebrate group.

1. Invertebrates
I. Sponges (Phylum Porifera)
• First animals to exist in multicellular form
• Simplest kind of animal
• Live in salty water
• Have few specialized cells, no tissues or organs
• Has 2 layers of cells w/ jelly-like substance in between
• Flagella direct currents of water through the pores in the sponge
• Reproduce in a special asexual way: one sponge can be cut into 10
pieces, each will become a new sponge
• They also reproduce sexually
• http://www.youtube.com/watch?v=laJgUrSsO_k

2. II. Hollow-bodied animals (Phylum Cnidaria)
• Bodies are big sacks
• Has one opening – food and waste goes in and out
• Are radially symmetrical (parts extend from a center like spokes of a wheel)
• Jellyfish has tentacles/arms with stingers around the mouth
• Corals have a hard covering called the exoskeleton
• Sea anemones

3. III. Worms
• Flatworms (Phylum Platyhelminthes)
- example : tapeworm
- are flat/ribbon-shaped/leaf-like
- are parasites, cause diseases
- have one digestive-system opening
• Roundworms (Phylum Nematoda)
- parasites, live in the intestines of animals
• Segmented (Phylum, Annelida)
- Example : earthworms
- Have 100 to 180 ring like segments
- Have head, tail, mouth, blood, and a tiny brain
- They move by means of muscles
- Have closed circulatory systems

4. IV. Spiny-skinned animals
• have thin skins, covered w/ spines
• Have tube feet to hold onto rocks and to open food
• Examples : starfish, se urchin, sand dollars
• Starfish pushes its stomach out, digests the food, and then
swallows the stomach
V. Mollusks
• Tasty food for people, also helpful
• Have soft bodies
• Most have shell coverings called the mantle (except squids and
octopus)
• Most live in salt water
• Snails move with the use of a muscular foot which releases a trail of
slime

5. VI. Arthropods