Sertularido.— The depths of the ocean are inhabited by innumerable zoophytes equally remarkable for the beauty of their appearance and the peculiarity of their structure; these are the Sertularice, whose arborescent sterns have so much the appearance of vegetable productions that they are still regarded by the uninformed as " sea-weeds." On putting a living specimen of a Sertularia (fig. 54) into a jar of its native element, and watching it attentively with the aid of a magnifying glass, its real nature becomes at once appa rent, and instead of being of vegetable origin, all the elegant ramifications of which it consists are found to be peopled with numbers of hydriform polypes, all actively employed in catching prey, and apparently ministering to the support of the general community formed by their aggregation.
The stem of a Sertularia consists of a hol low tube, composed of a flexible horny sub stance, diversely ramified in different species, in the interior of which is enclosed a soft animal substance, which constitutes the living portion of the zoophyte. At regular intervals every branch is studded with little cells, com posed of the same horny material as the general stem, in each of which is lodged a Hydra, or at least a polype similar to the Hydra in its general characters, the base of which is continuous with the central living pith that permeates the stem, which thus seems to be nourished by the hundreds of little polyrs that are constantly fishing for food.
At certain periods of the year, besides the polype-bearing cells, other horny receptacles are developed, called the ovigerous vesicles (fig. 55, h), in which the ova are produced.
The ovigerous vesicles are differently dis posed according to the species, sometimes arising from the branches of' the coralline, at others from the axillae formed by their sub divisions ; their shape likewise is very various, and sometimes they are covered with a little operculurn, or lid, which closes the orifice of the vase-like vesicle during the maturation of the reproductive gemmules, and at last opens so as to permit their escape. These gemmu liferous urns are, however, deciduous, and fall off after the development of the germs of reproduction is completed.
Such being the general structure of the Sertularidae, we must now proceed to examine more minutely their intimate organisation. The stem of the Sertularian is composed of two layers, of which the exterior (fig. 55, b),
or legunienlary layer, is of a dense horny tex ture, while the internal, or nutritive layer (fig. 55, a), is of a soft pulpy character according to the pattern peculiar to the species ; the tegu mentary layer expands at appointed distances into the polype-cells (fig. 55, g); and it is from this layer likewise during the reproductive season that the ovigerous vesicles are deve loped. The nutritive layer (fig. 55, a), it will be seen, not only lines the stem, but likewise penetrates into the polype-cells, where it be comes continuous with the body of the con tained polype, the structure-of which closely resembles that of the Hydra; it seems, in fact, to consist of nothing but a stomachal sacculus, the mouth of which is surrounded with con tractile tentacles, which are never, as errone ously stated by some writers, provided with vibratile cilia, such as are possessed by some more highly organised polypes. The nutr:ment elaborated in the digestive sacculi passes into the central cavity of the stem, in which an evident circulation of globules is apparent, somewhat analogous in its appearance to what is perceivable in the Chara and other transpa rent vegetables.
It is from the nutritive layer which lines the ovigerous vesicles likewise that the repro ductive gemmules are developed. These (fig. 55, d), as they gradually become separated from the nidus in which they are formed, retain their connection with the vital tissue of the nutritive layer, by the intervention of a kind of umbilical cord, until they are suffi ciently matured to allow of their escape. When this period arrives each gemmule is found to be covered over with vibratile cilia, by the action of which it detaches itself from the umbilical filament, and, escaping from the reproductive cell, swims away into the sur rounding element.
Here, by means of its cilia, it swims about, having much the appearance of a polygastric animalcule, until it finds a fit locality for its development, when it settles down, and, losing its locomotive organs, spreads out like a film ofjelly upon the supportmg body. The formation of its horny envelope then begins, fibres of which are first extended like the spreading root of a tree, so as to give a firm hold upon the basis for support ; and then the stem itself begins to shoot upwards, deve loping, as it ascends, the nutritive polypes and the horny cells in which they are individually lodged.