Characteristics of Phylum Cnidaria

Body Shape

• Most animals are bilaterally symmetrical, in that the left side mirrors the right side. Cnidarian animals, however, possess bodies with radial symmetry. Like a star or spokes on a wheel, the Cnidarian body is situated symmetrically around a central axis or point. It has the same shape from every side. Two body shapes predominate the phylum: Medusa shapes typical of the umbrella-shaped jellyfish; and the polyp form of the sea anemone, which is tube-shaped with a ring of tentacles around one end. Significantly, many species of this phylum go through a bi-phasic, or two-stage life cycle, with each animal existing as a polyp and as a Medusa at different times in its life cycle.

Simple Systems

• Considered a more primitive phylum in the animal kingdom, the Cnidarian has one orifice that serves as both mouth and anus and leads to the animal's gastrointestinal cavity. It lacks a central nervous system, internal organs and typical muscle cells. Instead, the animal uses a simple net of nerves covering its body to sense stimuli, and fibers that contract so the animal can capture food or move.

Toxins

• Although carnivorous, cnidarians do not hunt prey, but lie in wait for it, using their bodies as traps for unwary victims. They have specialized cells called cnidocysts on the surface of their body. Inside each cnidocyst is the nematocyst, a capsule containing toxins and a slender thread. In many Cnidarians, these cnidocyst cells propel the poison via a tiny thread with a barb attached that hooks onto the victim and injects the poison, which immobilizes the prey. Some of these toxins are poisonous or at least irritating to humans. For example, the sting of a jellyfish rarely causes death in humans, but it does hurt.

Subcategories

• The phylum Cnidaria is further divided into three subcategories. Anthozoan species, which include sea anemones and corals, exist as mostly sedentary polyps. Primarily Medusa and colony-forming species such as hydra belong to the Hydrozoan group, and the category Scyphozoa contains the jellyfish.