Cnidarian anatomy

thumb|Nomarski micrograph of a [[ruthenium red-stained nematocyst from Aiptasia pallida, the pale anemone. The red dye stains the polyanionic venom proteins found inside the capsule of this partially-discharged nematocyst.]]

Mesoglea refers to the extracellular matrix found in cnidarians like coral or jellyfish as well as ctenophores that functions as a hydrostatic skeleton. It is related to but distinct from mesohyl, which generally refers to extracellular material found in sponges.

thumb|right|alt=|Drawing of the statocyst system thumb|Statocysts (ss) and statolith (sl) inside the head of sea snail Gigantopelta chessoia

thumb|Pyrosoma atlanticum, a [[tunicate, is a colony of zooids]]

thumb|right|200px|Moon jellyfish (Aurelia aurita), with rhopalia visible in indentations of the bell's rim

simplest form of a nervous system: neurons are found spread apart, allowing the organism to respond to its environment

thumb|280x280px|Cross section of jellyfish. The gastrodermis is numbered 3. Gastrodermis (from Ancient Greek: , , "stomach"; , , "skin") is the inner layer of cells that serves as a lining membrane of the gastrovascular cavity in cnidarians. It is distinct from the outer epidermis and the inner dermis and is primarily associated with the ventral side of cnidarians. It is composed of specialized cells responsible for several vital physiological processes. The term is also used for the analogous inner epithelial layer of ctenophores.

thumb | right | alt=The coenosarc and polyps of Echinopora lamellosa | Coenosarc (blue) and polyps (green) of Echinopora lamellosa In corals, the coenosarc is the living tissue overlying the stony skeletal material of the coral. It secretes the coenosteum, the layer of skeletal material lying between the corallites (the stony cups in which the polyps sit). The coenosarc is composed of mesogloea between two thin layers of epidermis and is continuous with the body wall of the polyps. The coenosarc contains the gastrovascular canal system that links the polyps and allow them to share nutrients an

thumb|Diagram showing a coral polyp, its corallite, coenosarc and coenosteum thumb|Up : zoom on the skeletal cup of an Astrangia poculata|Astrangia coral; Down : view of the skeleton of the whole colony, showing all the coralittes. A corallite is the skeletal cup, formed by an individual stony coral polyp, in which the polyp sits and into which it can retract. The cup is composed of aragonite, a crystalline form of calcium carbonate, and is secreted by the polyp. Corallites vary in size, but in most colonial corals they are less than in diameter. The inner surface of the corallite is known as

Coenenchyme is the common tissue that surrounds and links the polyps in octocorals. It consists of mesoglea penetrated by tubes (solenia) and canals of the gastrodermis and contains sclerites, microscopic mineralised spicules of silica or of calcium carbonate. The outer layer of the coenenchyme is made of columnar or squamous epithelial cells, and can be covered in microvilli. The stiff projecting portion of coenenchyme that surrounds each polyp is usually reinforced by modified sclerites and is called the calyx, a term borrowed from botany. The solenia circulate nutrients throughout the coene

In corals, the coenosteum is the stony skeletal material secreted by the coenosarc, the layer of living material lying between the corallites (the stony cups in which the polyps sit). The coenosteum is composed of aragonite, a crystalline form of calcium carbonate, and is generally a spongy, porous material. Sometimes the coenosteum has ornamentation such as ridges and beads, visible as raised areas of the coenosarc. The coenosteum and corallites together are known as the corallum.