The Cnidarian or Coelenterate

The word celenterado derives from the Greek Koilos, which means 'compartment', 'cavity', and entheron, which concerns 'gut'.

The word cnidarian is derived from Greek. knidos and means 'stinging', 'burning'.

The organization of the body of the coelenterates

Coelenterates can come in two forms: polyps or jellyfish.


They have a cylindrical body and generally live fixed, for example on a rock. At its free end, they have tentacles around the mouth.


They have the body shaped like an umbrella. Its tentacles are distributed along the margin of the body, in the center of which is the mouth. They swim freely, although generally to a limited extent, or are carried by streams of water.

The body of the coelenterates has a single opening - the mouth. This opening contacts a cavity called digestive cavity.

The body wall is externally lined by the epidermis. Some epidermal cells are modified: they are the cnidocytes. Observe the scheme:

Each cnidocyte has one capsule - the nematocyst - which houses inside it a threaded filamentous tube carrying a stinging liquid. The nematocyst also contains a sensory eyelash that acts as a "trigger": when touched, the nematocyst "fires" the urticating filament and injects the venom into the body of prey or predators, for example, causing serious injury and even injury. even death. This requires the joint action of many cynidocytes. Thus, cnidocytes serve for the capture of food or for the defense of the animal.

When prey is captured, it is taken to the animal's mouth and reaches the digestive cavity. In this cavity, the food is partially digested and then absorbed by certain cells within which digestion is completed. That is why it is said that the digestion in the celenterates is extracellular (in the digestive cavity) and also intracellular (inside cells). Unused waste is eliminated through the mouth.