Phylum: Cnidaria

This phylum used to be called coelenterata, and is often referred to as such. The cnidarians (pronounce it without the "c") exhibit radial symmetry (their organs are arranged in a circle, or like spokes around a wheel) and many lack any sort of skeleton. Some members have stinging cells to stun their prey, and tentacles with which to draw their prey into their bodies.

Cnidarians are divided into three classes: hydrozoa, scyphozoa, and anthozoa.

Class: Hydrozoa

A hydrozoan, as an adult, attaches its base to an object, and extends its tentacles out into the water, resembling a tree. At this point it is called a polyp. A common freshwater member of this class is the hydra. An unusual member is the Portuguese Man O' War, which is a colony of specialized polyps forming a balloon-like float and hanging stinging tentacles—a Portuguese Man O' War can badly sting people who inadvertently swim into its tentacles.

Hydra. Society for Environmental Education.

Portuguese Man 'O War. NOAA.

Class: Scyphozoa

A scyphozoan, as an adult, floats freely with its tentacles and stinging cells hanging down. It is thus upside-down when compared to the adult (polyp) stage of most hydrozoans. At this stage in life it is said to be in the medusan stage. Members of this class are the true jellyfish. Jellyfish can be tiny, or up to ten feet across with tentacles over a hundred feet long. Most are harmless, but some have stinging toxins strong enough to quickly kill a person.

National Aquarium in Baltimore

NOAA

Class: Anthozoa

Anthozoans have only the polyp stage of their lives, and include the sea anemones and corals.

Sea anemones can often catch prey in their tentacles, but many also rely on a symbiotic relationship with photosynthetic protists for survival. Some sea anemones can grow up to three feet in diameter.

Corals are unusual in that many types form a hard exoskeleton of calcium carbonate (limestone). The soft-bodied creatures hide within these stony chambers and cluster together in great colonies. These colonies may survive for hundreds or even thousands of years, and form reefs which stretch for thousands of miles!

National Aquarium in Baltimore

National Aquarium in Baltimore

Sponges and Corals. NOAA.