There are eight MESENTERIC FILAMENTS in all Alcyonarian zooids. They have the appearance of thickenings of the free edges of the mesenteries. Two of them, called the "dorsal" mesenteric filaments, are straight when the anthocodia is expanded, and extend from the edge of the stomodaeum for a long distance down into the coelenteron of the zooid; the other six, called the "ventral" mesenteric filaments (_i.e._ the ventral and ventro-lateral and dorso-lateral), are usually short and are almost invariably slightly convoluted. The dorsal filaments are built up of columnar cells provided with long cilia, and have usually no gland cells, the others may show a few cilia but are principally composed of non-ciliated gland cells. When the bolus of food has passed through the stomodaeum it is seized by these ventral filaments and rapidly disintegrated by the secretion of its cells. The function of the dorsal mesenteric filaments is mainly respiratory. During life their cilia produce a current which flows towards the stomodaeum. On the ventral side of the {334}stomodaeum itself there is a groove called the "siphonoglyph" composed of a specialised epithelium bearing long powerful cilia. But the current produced by the siphonoglyph flows from the mouth downwards into the coelenteric cavity and is thus in the opposite direction to that produced by the dorsal mesenteric filaments. It is very probable that these two currents on the opposite sides of the zooids maintain the circulation of water in the deep-seated parts of the colony which is necessary for the respiration of the tissues. On each of the eight mesenteries there is a longitudinal ridge due to the presence of a band of retractor muscles. The position of these muscles on the ventral surfaces of the mesenteries only is one of the characteristic features of the sub-class (Fig. 148, and p. 329). They vary considerably in thickness and extent according to the power of retractility possessed by the zooids, but they never vary in their position on the mesenteries. [Illustration: FIG. 148.—Diagrammatic transverse sections of an Alcyonarian. A, through the stomodaeum; B, below the level of the stomodaeum. _DD_, Dorsal directive; _dlmf_, dorso-lateral mesenteric filament; _dmf_, dorsal mesenteric filament; _gon_, gonad; _Si_, siphonoglyph; _V.D_, ventral mesentery; _V.L_, ventro-lateral mesentery. The upper half of the section in B is taken at a higher level than the lower half.] The SKELETON of Alcyonaria may consist of spicules of calcium carbonate, of a horny substance frequently impregnated with calcium carbonate and associated with spicules of the same substance, or in _Heliopora_ alone, among recent forms, of a continuous crystalline corallum of calcium carbonate.