In the Edwardsiidea, Actiniaria, and Cerianthidea, that is to say in the animals popularly known as Sea-anemones, asexual reproduction does not commonly occur, but nevertheless a good many instances of it are now known in individual genera. In Actinoloba (Metridium), for example, Parker has described a case of complete longitudinal fission, and Duerden states that it occurs in the West Indian Anemones Actinotryx and Ricordea. A still more remarkable form of asexual reproduction known as transverse fission has been described in the genus Gonactinia.[[397]] In this case, the body of the Anemone becomes constricted in the middle, a circlet of tentacles is formed below the constriction, and division takes place. The upper half floats away with the original tentacles and stomodaeum and becomes attached by the base in another place; the lower half remains behind and develops a new stomodaeum, mesenteric filaments, and sexual organs. In some of the Actiniaria another form of asexual reproduction occurs, known as "Pedal laceration." In the common British Actinoloba, for example, so often kept in aquaria, the pedal disc sometimes spreads on the glass or rock upon which the animal rests, in the form of a thin membrane or film of an irregular circular shape, nearly twice the diameter of the column. As the Anemone glides along, the film remains behind and breaks up into a number of hemispherical droplets, which in a few days develop tentacles, a mouth, mesenteries, and the other organs of a complete and independent Anemone. A similar method of reproduction has been observed in several species of Sagartia. A true process of discontinuous gemmation has also been observed in Gonactinia, in Corynactis, and in Actinoloba.
Fig. 164.—Longitudinal fission of Actinoloba. (After Agassiz and Parker.)
In the Madreporaria, Zoanthidea and Antipathidea, the usual method of reproduction to form the colonies is continuous gemmation. The new zooids that are added to the colony as it grows arise as buds, either from the superficial canals of the coenenchym, or from the base or body-wall of the older zooids. In these cases the young zooids acquire the same number of mesenteries, and the same characters of the stomodaeum as the original parent. Some further particulars of asexual reproduction in the Madreporaria are given on p. [387].
The sexual reproduction of a great many species of Zoantharia has now been observed. The eggs are, as a general rule, ripened in batches, and fertilisation is effected before their discharge from the body. In some cases the sexual condition is seasonal. In temperate climates the generative organs ripen in the spring and summer months, and remain small and relatively inconspicuous in the colder weather; but British Sea-anemones, when kept in an aquarium and regularly fed, will breed nearly all the year round. The corals of the tropics living in warmer water of a more regular temperature show considerable variety in their breeding habits. Thus Duerden found that colonies of Favia, Manicina, Siderastraea and Porites are fertile at nearly all times, whereas colonies of Madrepora, Orbicella and Cladocora were rarely so. In nearly all cases the fertilisation is effected, and segmentation of the ovum occurs within the body of the parent, the young Zoantharian beginning its independent life as an oval or pear-shaped ciliated larva.
There are a great many cases among the Actiniaria in which the embryos are retained within the coelenteron, or in special brood pouches of the parent (p. [379]), until a stage is reached with twelve or more tentacles.
The oval or pear-shaped larva swims about for a few days or hours, and then settles down on its aboral end. In swimming, the aboral end is always turned forwards. In the larva of Lebrunia coralligens and Rhodactis sancti-thomae, a distinct sense organ has been observed upon the aboral extremity, and a similar but less distinct organ on the larva of Actinia equina. These organs are of considerable interest, as they are probably the only specialised sense organs known to occur in the Zoantharia.
The larvae of Zoantharia present, as a rule, very little variation from the type described, and live but a short time if they fail to find a suitable place for fixation. The colour is usually white and opaque, but in some species the endoderm may be coloured yellow by Zooxanthellae (cf. pp. [86], [125]).
The larvae of the Cerianthidea, however, are remarkable and exceptional. After the larva of these animals has passed through the gastrula stage, a certain number of mesenteries and tentacles are formed, and it rises in the water to live a pelagic life of some duration. This larva is known as Arachnactis, and is not unfrequently found in the plankton.
The character of the food of the Zoantharia varies with the size of the zooids, the occurrence of Zooxanthellae in the endoderm, and local circumstances; but in general it may be said to consist mainly of small living animals.