Hydra – Wall Anatomy

The body wall of a hydra is composed of two main layers separated by a non-cellular, membranous mesoglea. The outer layer, known as the ectoderm, covers the body, while the inner layer, called the endoderm or gastrodermis, lines the body cavity. Both the ectoderm and endoderm are single layers of cells. The ectoderm consists of epithelial-muscle cells with projections firmly attached to the mesoglea, creating a structure known as epithelial muscles. These cells secrete mucus that covers the hydra's body, providing protection and lubrication. Among these cells are sensory cells, each equipped with a sensitive flagellum and linked to neural cells, which together form a sensory network. The sensory and neural cells are connected to specialized stinging cells. Each stinging cell is covered by a lid and contains a highly sensitive sensor. When this sensor is triggered, the lid opens, and the cell explosively discharges its contents. Hydras possess three types of stinging cells. The first type, called penetrants, are equipped with a harpoon-like spear and backward-facing spines, designed to pierce the body of prey or an attacker. Once penetration occurs, a stinging tube releases a toxic liquid through the spear-like tip. The second type, volvents, coil around the target’s body after discharge. The third type, glutinants, release sticky filaments that help immobilize prey or repel threats. The endoderm is made up of nutritive-muscular cells that serve multiple functions. Some of these cells produce digestive enzymes to break down food, while others engulf food particles for further digestion. Scattered neural cells are also present within the endoderm.