The order Actiniaria contains a large number of families, presenting a great variety of external form and of detail in general anatomy. The definitions of the families and their {380}arrangement in larger groups have presented many difficulties, and have led to considerable differences of opinion; and even now, although our anatomical knowledge has been greatly extended, the classification cannot be regarded as resting on a very firm basis. The families may be grouped into two sub-orders:— SUB-ORDER 1. ACTINIINA.—The tentacles are simple and similar, and there is one tentacle corresponding to each intermesenteric chamber (endocoel). SUB-ORDER 2. STICHODACTYLINA.—The tentacles are simple and similar, or provided with teat-like or ramified pinnules. One or more tentacles may correspond with an endocoel, and there may be two kinds of tentacles (marginal and accessory) in the same genus. SUB-ORDER 1. ACTINIINA. FAM. 1. HALCAMPIDAE.—This family is clearly most closely related to the Edwardsiidea. There are, however, twelve complete mesenteries of the first cycle, and a second cycle of more or less incomplete mesenteries. The tentacles are usually twelve in number, but may be twenty or twenty-four. There is no pedal disc, but the base is swollen and rounded or pointed at the end. The genus _Halcampa_ includes a considerable number of small species occurring in the shallow waters of the temperate northern hemisphere, and of the Kerguelen Islands in the south. Three British species have been described, of which _Halcampa chrysanthellum_ alone is common. The larva with eight tentacles and eight mesenteries has been found living on the Medusa _Thaumantias_. _Peachia_ is a genus containing Anemones of much larger size (10-25 cm.). It is remarkable for the very large siphonoglyph on the ventral side of the stomodaeum, prolonged into a papillate lip projecting from the mouth called the "conchula." The genera _Scytophorus_ from 150 fathoms off Kerguelen and _Gyractis_ from Ceylon, although showing some remarkable peculiarities of their mesenteric system, appear to be closely related to this family. _Ilyanthus mitchellii_ is a large Anemone with a vesicular base, forty-eight tentacles and mesenteries, occurring in the English Channel, but it is not very common. It is usually {381}placed in a separate family, but is in many respects intermediate in character between the Halcampidae and the Actiniidae. FAM. 2. ACTINIIDAE.—This family contains some of the commonest British Sea-anemones. There is a large flat pedal disc by which the body is attached to stones and rocks. The body-wall is usually smooth, and not perforated by cinclides. The edge of the disc is usually provided with coloured marginal tubercles. There are no acontia.