1. Not the one from Hercules!!
The Hydra Lab
Not the one from Hercules!!

2. The Cnidarian

3. Cnidarians (formerly coelenterates) have two cell layers, tentacles armed with stinging cells, a single-ended digestive cavity, and a nerve net that allows some general responses to environmental stimuli. In 1997, scientists succeeded in cloning a mammal, but cnidarians have been in the cloning business for close to 600 million years. One of the best examples is seen in the common fresh-water Hydra, a cnidarian that reproduces by "budding."
Class Hydrozoa Hydra captures microcrustaceans, swallowing and digesting them in its gut. It uses two methods of digestion. One is primitive — engulfment of small food particles that then are digested in vacuoles located in the phagocytic cells lining the gut (the same methods used by amoebas and other phagocytic protists). Extracellular digestion also occurs. This process is initiated by hyfrolytic enzymes produced by gland cells in the gut lining. The absorbed nutrients diffuse to the outer layer of cells without the requirement of a circulatory system. Undigested material is regurgitated through the mouth, one shortcoming of a single-ended gut. With minor variations, this is the method of digestion and food transport found in all cnidarians.

4. Stinging cells inject paralyzing poisons and enzymes that begin the process of digestion even before the prey is swallowed. Trigger hairs, brushed by a daphnia's antennae, fire out their darts and lashes. Note the violent response to contact shown by Daphnia and the red copepods used in feeding trials. As the poisons take effect, the prey rapidly looses its ability to resist.
Hydras move by looping, basically somersaulting to a new location.
The green hydra harbors symbiotic algae in its tissues. Although it receives some nutrients from its photosynthetic guests, Chlorohydra eats a balanced diet, gorging on microcrustaceans when available.
Budding new individuals from a few cells located on Hydra's lower body is a natural form of cloning. The individuals produced are identical genetically to their mother or father. Sexual exchanges shuffle the genetic deck, usually just before winter sets in. Fertilized eggs withstand freezing, producing a new hydra in the spring.

5. Longitudinal Section

6. Cross Section
Here we see a cross section through the body of a Hydra sp. Note that the body wall is only made of two layers, the outer epidermis, and the inner endodermis.

7. Ingesting a Daphnia
                                                            

8. Do they ever stop eating?

9. The Hydra Budding
Here you can see a photo of a Hydra sp. with two asexual buds.
Chlorohydra viridissima

10. Sexual Organs
Here you can see an image of a Hydra sp. with ovaries growing in the body wall.
In this image you can see the testes growing in the body wall.