Hydra – Wall Anatomy

Body wall of a hydra consists of two main layers separated by non-celular membraneous mesoglea. Outer layer covering body is called ectodermis, inner layer entodermis (gastrodermis). Ectodermis and entodermis are single-layered.

Ectodermis is made of epithelial-muscle cells with their projections firmly attached into mezoglea creating thus epithelial muscles. There cell excrete mucus covering the body. Among them are scattered sensory cells with sensitive flagellum and connection to neural cells. Sensory and neural cells are attached to stinging cells. Stinging cells are closed by a lid and also have a very sensitive sensor. When the lid opens as a reaction to the sensor irritation, it explosively releases the content of the cell that shoots out. Hydras have three types of stinging cells. Penetrants with a harpoon like spear and backward-facing spines that are designed to penetrate the body of a victrim or an attacker. Through spear-like tip enters the body stinging tube releasing stinging liquid. Another type, volvents loop around the body after discharging and the last type, glutinants release sticky filaments.

Entodermis is made of nutritive-muscular cells diversified into several types. Some of them produce digestive enzymes while other swallow particles of food. Among them are scattered neural cells.