Phylum Cnidaria

Body Structure

external image cnidariaDiagram.jpeg Cnidarians are invertebrates and their bodies of cnidarians are diploblastic. That means they have two layers. These two layers are the ectoderm and the endoderm. In between those two layers is the mesoglea which serves as glue to hold the two layers together. Cnidarians have only one opening that serves as both a mouth and an anus. Nematocyst-containing tentacles surround the mouth, which help in the process of capturing food. Cnidarians have specialized tissue to go through basic bodily functions. Therefore they have no need for a heart or brain. They have two layers called an outer epidermis and an inner feeding surface.

Obtaining Food

Cnidarians use nematcysts to capture their food. Cnidarians also use stinging cells to catch their food. They inject a toxin into their prey to disable them, allowing them to eat their food in peace. Then the Cnidarian uses their tentacles to grab their food and put it in their mouth which is located either on the top or bottom of their body. When they are digesting their food, the food goes to a hollow cavity where it is turned into mechanical energy.


sea anemone
The cnidarians use both sexual and asexual reproduction. They use budding when reproducing asexually. Some cnidarians reproduce sexually by releasing egg and sperm into the water. The eggs will be fertilized by sperm and develop into a larva called planula. As these planula mature they will develop into a medusa. Medusa's also have both asexual and sexual stages.


There are two different types of cnidarians. The two types are polypoid and medusoid. Polypoid are attached to the surface, were as medusoids can swim freely. Medusoids don't move they float with the water waiting for its prey to come to him.


About 130 live in the Sydney Harbor and there are over 10,000 species of cnidarians worldwide. Over thousands of years they have lived in coral reefs but they have found some of the reef in Sydney, Australia. They have very sharp stingers which allows them to protect themselves from predators and they are also very light which makes it easier for them to move.

Importance to Humans

Some cnidarians are edible, and are often used in Eastern Asian food. A large number of the islands humans live on today can be traced back to the carcasses of dead cnidaria. In addition to that, jewelry has been made from colorful coral since prehistoric times.