Hydra is commonly called fresh water polyp. It is found in fresh water ponds, ditches and pools. The hydra has a tubular body. It is a fixed animal. Its proximal end attaches to the substratum is called basal disc. The distal end bears and mouth. A conical elevation below the mouth is called hypostome. The mouth is surrounded by tentacres which help in lomotion and food capture. The mouth opens into a central cavity called gastrovascular cavity or coelenterons.
The coelenterons is meant for extracellular digestion and circulation of food. The body wall of hydra comprises two layers of cells. The outer cell layer is called ectoderm and inner endoderm.In between these two layers, there is a non-cellular jelly-like substances called mesogloea. The mesogloea is secreted by both ecto and endoderms.
The epidermis comprises of seven types of cells-Epitheliomuscular cell, interstitial cell, gland cell, enidoblast, nerve cells, and sensory cells. The enidoblast contains a sac like organelle called nematocyst. The nematocyst contains poisonous substances called hypnotoxin. The enidoblasts are organs of offence and deffence in hydra.
The interstitial cells are totipotent cells which give rise other types of cells including germ cells. The endodermis consists of five types of cells such as nutritive cells, interstitial cells, gland cells, nerve cells and sensory cells.Hydra reproduces asexually forming buds and sexually through gonads (testis and ovary). The gonads develop from the ectoderm.