Sponges have long been considered the most primitive animals on Earth because their body plan is much like what scientists would expect for an early multicellular organism. Two lines of recent evidence have strengthened this hypothesis.

• Sponge fossils more than 570 million years old were found in Australia, making sponges one of the most ancient groups of known animals.
• Molecular evidence confirms that sponges are closely related to a group of protists called choanoflagellates. Choanoflagellates are very similar in size and shape to certain cells found within a sponge. These protists are considered the most likely ancestors of all animals.

• Sponges lack muscle and nerve cells.
• They are sessile, meaning they are unable to move from where they are attached.
• Sponges attach to hard surfaces.
• They secrete toxic substances that prevent other sponges from growing into their area and also protect them from hungry predators and parasites.
• Some of these chemicals have been used in the development of medicines to treat forms of cancer such as lymphoma.

Sponges reproduce both sexually and asexually.

• In some species, eggs and sperm are released into the water and fertilization occurs there.
• In other species, sperm is released into the water and the egg is fertilized within the female sponge.
• The fertilized egg develops into a free-swimming larva that attaches to a surface, where it remains and develops into its adult form.

• Some sponges reproduce asexually by budding.
• Buds break off from the adult sponge and float in the water until they attach to an underwater surface.
• They grow into their adult form once attached.

• Sponges do not have mouths. Their cells are arranged around a network of channels that let water flow directly through the sponge's body.
• Water is pulled into the sponge through tiny pores in its body wall.
• Used water is ejected from a larger hole at the top of the sponge called the osculum.
• Most of the thousands of known sponge species are marine filter feeders. Filter feeders eat by straining particles from the water.
• Sponges can be found in many colors and shapes. Some are shaped like tubes, while others lie flat against the ocean floor.
• All sponge bodies are made up of two layers of cells that cover a framework of collagen-like fibers called spongin.
• The skeleton is usually reinforced with hard calcium- or silicon-based crystals called spicules.
• While sponges do not have tissues, they do have several types of specialized cells.

• In contrast to sponges, cnidarians (ny-DAIR-ee-uhnz) can move.
• A jellyfish pulsing through the water and an anemone waving its tentacles make deliberate movements using simple nerves and muscles.

Cnidarians have two body forms: the polyp and the medusa, both shown in FIGURE 3.3.

• Polyps (PAHL-ihps) are cylindrical tubes with mouth and tentacles facing upward. This form is characteristic of cnidarians such as corals.
• Medusas are umbrella-shaped, with their mouth and tentacles on the underside. This form is characteristic of free-swimming cnidarians such as the jellyfish.
• Many cnidarian species alternate between the two forms during their life cycle.
• Both polyps and medusas have radial symmetry — a characteristic of all cnidarians.

A cnidarian may reproduce both asexually and sexually during its life cycle.

• Polyps reproduce asexually by budding.
• This method produces genetically identical offspring.

• In the medusa form, cnidarians reproduce sexually by releasing gametes into the water.
• The fertilized egg develops into a free-swimming larva called a planula.
• The planula then develops into the polyp stage.

• Cnidarian bodies have two tissue layers separated by a non-cellular jellylike material called mesoglea (MEHz-uh-GLEE-uh).

• A nematocyst (NEHM-uh-tuh-sist) is a capsule containing a thin, coiled, harpoon-shaped tubule with a poisonous tip, found in both sea anemones and jellyfish.
• Nematocysts usually do not fire on contact unless a chemical signals the presence of prey or a predator.
• When they fire, nematocysts uncoil rapidly to spear and poison prey.

• Prey captured by nematocysts on the tentacles are stuffed through the animal's mouth into a saclike digestive space called the gastrovascular cavity.
• The cavity is lined with the cnidarian's inner tissue layer, which has cells that secrete digestive enzymes and absorb nutrients.
• Cnidarians do not have an anus. Wastes are pushed out through the mouth.
• The gastrovascular cavity also moves oxygenated water to internal cells.
• When the animal's mouth is closed, water in the cavity becomes pressurized and provides skeletal support to the tissue, similar to a balloon full of water.
• Muscular contractions can work against the pressurized fluid and change the animal's shape.

There are four major groups, or classes, of cnidarians. Each class is defined in part by which body form is dominant during the animals' lives.