Coelenterates

The word coelenterate means �hollow body cavity�.

All coelenterates contain a central cavity with  only one opening .

There are two general body forms found among the coelenterates.

The Polyp form is usually sessile and has a cylindrical body with a mouth and tentacles at the upper free end. 

- The habitat of coelenterates is salt water but some of them live in fresh water.

- They do not have a complete nervous system but they have  nerve net .

- Coelenterates have a sac like gut which serves as mouth and is surrounded by tentacles.

- Their respiration method is similar to that of sponges. They respire only by means of diffusion across the cell membrane.

- Although most coelenterates reproduce sexually, some of them reproduce by budding

- Coelenterates have two definite tissue layers; an ectoderm and an endoderm.

The structure of a hydra is a typical example of a coelenterate.

A group of tentacles surrounds the mouth that opens into the cavity.

The body wall of coelenterates has an outer layer (ectoderm), an inner layer (endoderm) and

 an intermediate layer, the mesoglia. The intermediate layer contains cells and jelly like substances.

The gastro vascular cavity serves both as a circulatory and digestive system.

It is surrounded by an inner layer of the body that is made up of secretory cells and columnar cells.

These cells have the ability to engulf food particles. They are digested and then diffuse form the body.

SUMMARY       Coelenterates

-Most Coelenterates have stinging cells called nematocysts on their tentacles.

   Coelenterates sting and capture their prey, which they digest in the central body cavity.
   Waste products are released through the mouth.

1. Digestive cavity
2. Two life stages:
   attached polyps and
   free-swimming medusa

3. Tentacles with
    stinging cells
4. Radial symmetry
5. Nerve net
6. Aquatic organisms

   Hydra

The hydra is a simple multicellular animal about 5 millimeters long from the tip of its tentacles to its base.

      CORAL

 Corals, like all coelenterates, are soft-body organisms.but corals use minerals in the water to build a hard protective covering of limestone.

       Coral

�    The mouth of this coral is surrounded by tentacles.

�    What is the function of tentacles?

Since algae need sunlight to
 make food , corals must live in shallow water where sunlight can reach them.

  Sea Anemones

Sea anemones look like underwater flowers. However, the �petals� are really tentacles, and their brilliant coloring helps attract passing fish.

 When a fish passes over the anemone`s stinging cells,the cells poison the fish.

At the same time, the clownfish serve as a kind of living bait for the anemone. Other fish see the clownfish, come closer, and are trapped by the anemone.

Jellyfish

    Jellyfish

If you saw a jellylike cup floating in the water, recognize this coelenterate, the jellyfish.

In fact, even when they are broken  up into small pieces, the stinging cells remain active and can sting a passing swimmer who accidentally bumps into them.

Aurelia is a common jellyfish. Its life cycle includes both medusa and polyp forms.The jellylike body of the medusa is the form commonly seen on beaches.

Protective tentacles hang from the edge of the umbrella-like body

The sexes are separate in Aurelia, but the male and female look alike.

Some sperm cells enter the gastro vascular  cavity of a female medusa, where fertilization occurs.

   Early development occurs while the zygote is attached to the female. The zygote develops into a small, oval-shaped, ciliated larva called a planula.

The planula is free swimming for some time.It then becomes attached by one end to a rock. 

The larva  develops a mouth and tentacles at the unattached  end  and becomes a polyp.

The polyp grows, eventually reproducing asexually to form medusa. This occurs in the fall and winter.

The medusa stage reproduces sexually by the production of egg and sperm, and it gives rise to the polyp stage.