The hard calcareous substance to which the name coral is applied is the supporting skeleton of certain members of the _Anthozoa_, one of the classes of the phylum Coelentera. The most familiar Anthozoan is the common sea-anemone, _Actinia equina_, L., and it will serve, although it does not form a skeleton or _corallum_, as a good example of the structure of a typical Anthozoan polyp or zooid. The individual animal or zooid of _Actinia equina_ has the form of a column fixed by one extremity, called the _base_, to a rock or other object, and bearing at the opposite extremity a crown of _tentacles_. The tentacles surround an area known as the _peristome_, in the middle of which there is an elongated mouth-opening surrounded by tumid lips. The mouth does not open directly into the general cavity of the body, as is the case in a hydrozoan polyp, but into a short tube called the _stomodaeum_, which in its turn opens below into the general body-cavity or _coelenteron_. In Actinia and its allies, and most generally, though not invariably, in Anthozoa, the stomodaeum is not circular, but is compressed from side to side so as to be oval or slit-like in transverse section. At each end of the oval there is a groove lined by specially long vibratile cilia. These grooves are known as the _sulcus_ and _sulculus_, and will be more particularly described hereafter. The elongation of the mouth and stomodaeum confer a bilateral symmetry on the body of the zooid, which is extended to other organs of the body. In Actinia, as in all Anthozoan zooids, the coelenteron is not a simple cavity, as in a Hydroid, but is divided by a number of radial folds or curtains of soft tissue into a corresponding number of radial chambers. These radial folds are known as _mesenteries_, and their position and relations may be understood by reference to figs. 1 and 2. Each mesentery is attached by its upper margin to the peristome, by its outer margin to the body-wall, and by its lower margin to the basal disk. A certain number of mesenteries, known as complete mesenteries, are attached by the upper parts of their internal margins to the stomodaeum, but below this level their edges hang in the coelenteron. Other mesenteries, called incomplete, are not attached to the stomodaeum, and their internal margins are free from the peristome to the basal disk. The lower part of the free edge of every mesentery, whether complete or incomplete, is thrown into numerous puckers or folds, and is furnished with a glandular thickening known as a _mesenterial filament_. The reproductive organs or gonads are borne on the mesenteries, the germinal cells being derived from the inner layer or endoderm.