122 TWO-LAYERED ANIMALS— PHYLUM COELENTERATA 



pretreatment. The mouth is the only opening to the gastrovascular 

 cavity in hydra — hence it must serve the dual function of a mouth and 

 an anus. Indigestible wastes are egested through the same opening 

 that is used for ingestion. 



Some hydra are green in color because of the presence of certain 

 one-celled algae within their cells. These algae apparently live here 

 as "guests" and seem neither to harm nor to help their hosts to any 

 appreciable extent. Such associations are sometimes called commensal- 

 ism. 



Respiration and excretion in hydra are taken care of by simple dif- 

 fusion as described for the sponges. Practically every cell of the 

 hydra's body is exposed to water, either on the outside of the body or 

 within the gastrovascular cavity ; hence diffusion is a simple matter. 

 A cross-section of the body of hydra shows only two layers of cells, the 

 outer one being the epidermis and the inner one being the gastrodermis. 

 Between these there is a thin layer of a jelly-like substance known as the 

 mesoglea. 



With respect to reproduction, hydra is very much like scypha. It 

 reproduces asexually by budding; a little bump will appear about one 

 third of the body length up from the base which soon sprouts tentacles 

 and within two or three days is ready to cut itself loose from the parent 

 as a fully developed animal. Buds do not remain attached to the parent 

 and form a colony as they did in the sponge. 



In sexual reproduction gonads are formed ; the word gonad refers to 

 an organ that produces gametes. The male gonads, or testes, are formed 

 near the oral end and release the sperms through little nipples at the tip 

 of the organs out into the surrounding water. The sperms swim to the 

 egg which is produced in the female gonad, or ovary, usually found 

 about a third of the way up from the base, and one of them will unite 

 with it to form the zygote. This zygote drops loose from the parent and 

 develops a hard protective wall around itself and may live through the 

 winter in this condition while the other forms are killed. Sexual repro- 

 duction occurs most frequently in the fall, so that these resistant forms 

 may be produced to tide them over the winter. Most hydra still seem 

 to be hermaphroditic and can produce both testes and ovaries even 

 though they may not produce both at the same time. 



Regeneration in hydra is about the same as it was in scypha. In 

 fact, hydra gets its name from its regenerative powers. According to 

 Greek myth, there was an old sea serpent named Hydra that Hercules 

 went out to slay. The serpent had nine heads and if any one of these 

 was cut off two new ones would grow in its place. Our little animal 

 under consideration was given the same name because its tentacles have 



