1. Hydra is a bivalve. Its body wall consists of epidermis and gastrodermis. Between the epidermis and the gastrodermis are the acellular mesoglia.
2. Interstitial cells in the body can transform into any other cell.
3. Nervous system emerges from nerve cells to form neural network. It is widespread throughout the body. Dr. Siddique Publications
4. Cilenteron or gastrovascular cavity is present in the body. It acts as a body cavity and digestive tract.
5. Open to the outside through the gastrovascular orifice.
6. Intracellular and extracellular digestion takes place in the body.
7. Absorption and excretion occur in the process of diffusion and absorption.
8. Division of labor is observed for performing physiological and mechanical tasks.
9. Asexual reproduction and sexual reproduction occur in the body.
Hydra is called the simplest multicellular animal due to the above characteristics.