Cnidarian: What They Are, and Their Different Growth Stages

Cnidarian is a phylum which includes many different and diverse organisms with brilliant colors, and unusual shapes. For example, delicate jelly fish floating in the ocean are in the phylum cnidarian. While Cnidarians can be found all over the world, most species in this phylum can only be found in the sea.

Generally, Cnidarians are classified as soft-bodied animals with stinging tentacles which are arranged in a circle around the mouth. While Cnidarians are rather complex creatures and organisms as well, they have radial symmetry, and specialized cell tissues. Cnidarians have two very special and different life cycles that include the polyp which is a sessile flowerlike stage, and the medusa which is a motile bell-shaped stage.

While they have two different stages in life, both stages are actually rather similar. For example, both stages have the same organs and parts - mouth, tentacle, epidermis, mesoglea, gastroderm, and gastrovascular cavity.

In both stages, the body wall surrounds an internal space known as the gastrovascular cavity which is where the digestion takes place. The outer wall of these two stages consists of the same three layers: epidermis, mesoglea and gastroderm. Epidermis is the outer layer of the organism while the gastroderm is the later that covers the inner surface, and lines the gastrovascular cavity. The mesoglea is the thin non-cellular membrane or jelly like material that is between both the gastroderm and epidermis. This layer is thicker on the medusa which is the motile stage.

The polyp will generally stick itself and attach itself to a surface while the medusa is free-living in the sea. Both forms of the cnidarians rely on their tentacles to reach out and capture food. The medusa also uses the contraction and extension of the tentacles to propel them in the water, and move around.