AGE.—It is known that Sea-anemones kept in an aquarium and regularly fed will live for a considerable number of years without showing signs of weakness or failing health. Dalyell kept in an aquarium a specimen of _Actinia mesembryanthemum_, which lived for sixty-six years and then died a natural death; and specimens of _Sagartia_, still living, are known to be about fifty years old.[399] The unnatural conditions of life in an aquarium may have favoured the longevity of these specimens, and it would not be reasonable to conclude from these records that the average life of a full-grown Anemone on the rocks is more than thirty or thirty-five years, and perhaps it is a good deal less. As regards the Madreporarian Corals, we know but little concerning their duration of life. An examination of any living coral reef is sufficient to convince an observer that the power of asexual reproduction of the colonial forms is not unlimited; that colonies, like individuals, have a definite span of life, and that they grow old, senile, and then die a natural death if spared in their youth from accident and disease. Mr. Gardiner has calculated that the duration of life in solitary Corals like _Flabellum_ is about twenty-four years, in colonial forms such as _Goniastraea_, _Prionastraea_, _Orbicella_, and _Pocillopora_, from twenty-two to twenty-eight years. ORDER I. EDWARDSIIDEA. This order contains only a few genera and species of small size living in shallow water in various parts of the world. In external features they closely resemble several genera of the Actiniaria, particularly those belonging to the family Halcampidae. The distinguishing character of the order is to be found in the system of mesenteries. In all the species only eight mesenteries are complete, namely, the first two pairs of protocnemes, and the two pairs of directives (Fig. 163, 2), {376}and these usually support such large and powerful muscle-bands that they appear to be the only mesenteries present. A careful examination of transverse sections, however, reveals the fact that other mesenteries are present. The fifth and sixth pairs of protocnemes seem to be invariably represented, and two or three pairs of metacnemes can also be traced in some species.