Cnidarians are diverse and come in many shapes and sizes but there are some basic features of their anatomy that most share in common. Cnidarias have an internal sac for digestion which is called the gastrovascular cavity. The gastrovascular cavity has only one opening, a mouth, through which the animal takes in food and releases waste. Tentacles radiate outward from the rim of the mouth.
The body wall of a cnidarian consists of three layers, an outer layer known as the epidermis, a middle layer called the mesoglea, and an inner layer referred to as the gastrodermis. The epidermis contains a collection of different types of cells. These include epitheliomuscular cells which contract and enable movement, interstitial cells that give rise to many other cell types such as egg and sperm, cnidocytes which are specialized cells unique to cnidarians which in some cnidarians contain stinging structures, mucus-secreting cells which glandular cells that secrete mucus, and receptor and nerve cells which collect and transmit sensory information.