Hydras

What are they?

One of the best known of the hydrozoan family is the hydra. This tiny animal has no medusoid stage and spends it's life as a polyp. But hydra is not a typical hydrozoan. Most spend their life as a polyp and a jellyfish sort of animal. They life part of their life in each stage.

The hydra's are small aquatic animals that are the simplest of multicellular animals. They have two layers of cylinders . They can be from half a centimetre to one and a half, or more centimetres. They are hollow, closed or rather rounded at one end, and have tentacles at the other end.

How do they live?

There are many of the group that are colonial. Some are branched colonies, while others are huge structures. Others form floating colonies, often mistaken for jellyfish. They are often composed of many individuals.

With more than 2500 species, this group is usually unnoticed, mainly as they are in branched groups, mistaken for seaweed. They often attach themselves with rocks or other substrates. Some colonies encrust hard ones, while other form feather shaped colonies. One group - Hydractinia, form crust on shells of hermit crabs, while as it protects the crab from predators. Other have gas-filled floats which is like the gas bag in fishes.

Most of the group are marine, but some have adapted to marine life like the hydra's and a few jellyfish.

Another feature found in Hydrozoa is colonial organization, though there are some that lead the life like a hermit - alone.

The tentacles are used to capture food. They also have a sting to stun prey ( or escape from becoming one ). They move by gliding or rotating ( well, maybe not so dramatic ). Instead, they may form a loop, bring the top to the bottom and vice versa, and then repeating the process. It goes like this - loop, straighten with head down and bottom top, loop, come back to original position. Sometimes they cling to floating material and float lazily

It's body has two layers - ectoderm, endoderm ( ecto - outside, endo - inside ) with a jelly in-between. Both have fibers, in place of muscles. Interstitial cells, scattered in both layers, give rise to nerve threads.

Each ovary of a hydra has one egg, the testis a sperm. The eggs develop in the body, and come out through the body like a bud of a flower from the plant. It grows full size, and breaks off. Many can grow from one hydra.

Red Alert, prepare defence systems.

To stay alive ( which is the main aim of any animal ), the hydra has poisonous structure in the ectoderm ( makes sense again. If it was in the inside, the hydra might rupture itself in defence, and the mechanism will be called suicide mechanism ). These cells are nematocysts or stinging cells. The small animals ( mostly smaller than itself ) are brought in as food by the tentacles. They are engulfed by pseudopodia of some cells and are digested. When a hydra is cut into roughly large pieces, each one becomes a hydra.