Re: How do hydra maintain homeostasis?

Catherine,

You are correct that hydras are structurally simple.  They belong to a group 
of animals called the Cnidaria, which also includes sea anemones, corals, 
and jellies.  All of these animals have very simple bodies, but they still 
manage to maintain homeostasis and carry out all of life's activities.

Digestion: Cnidarians have a two-way gut, which means that the gut has a 
single opening that serves as both mouth and anus.  In other words, food 
goes in and waste leaves from the same opening.  Hydras catch food with the 
stinging cells in their tentacles, and bring it into the mouth.  Food is 
digested in the gut, nutrients are absorbed into the body, and wastes are 
ejected out through the mouth.

Gas exchange: Gas exchange occurs across the thin epithelium (body wall).  
The gut also serves as a circulatory system that distributes nutrients 
through the body, so there is no separate circulatory system with a heart or 
blood vessels.

Excretion: Nitrogenous wastes also diffuse across the body wall.  In hydras, 
which live in freshwater, excess water (continuously diffusing into the body 
tissues, which are more concentrated than the surrounding environment), is 
expelled through the mouth.  This manipulation of water concentration in the 
body is called osmoregulation.

Nervous system: As you mentioned in your question, hydras have a diffuse 
nerve net that coordinates movement and integrates sensory information.

References:
Pearse and Buchsbaum. 1987. Living Invertebrates. Blackwell/Boxwood Press.
Ruppert and Barnes. 1994. Invertebrate Zoology. Saunders College Publishing.