This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(231). According to the observations of Loven on the reproductive process in Sertularia, the first appearance of the reproductive germ is a slight elevation (derived from the central mass contained in the ovarian vesicle), in the centre of which an active circulation of nutritious globules seems to be concentrated. This protuberance gradually enlarges and assumes a spherical form, the part whereby it is attached to the central mass becomes constricted, and at the same time its cavity becomes enlarged, and divided into several compartments.
(232). Upon the outer aspect of the newly-formed germ a little spherical body may be detected, composed of coloured granular substance, in which a circular transparent spot speedily becomes perceptible.
(233). A delicate translucent capsule envelopes the parts described above, which, after a time, exhibits at its upper and outer surface a circle of minute elevations. This capsule Loven regards as the body of a female polyp, of which the little elevations are the rudimentary tentacula; and its contents manifestly constitute an ovum, enclosing a Purkinjean vesicle. Several of these ova are formed in the ovarian vesicle, presenting different degrees of development, the upper ones being the most advanced in growth. In proportion as each ovum increases in size, the original sacculus, which is merely a prolongation of the central living substance of the polypary, and which at first formed the larger part of the germ, becomes proportionally smaller, owing to the rapidly-increasing dimensions of the ovum, and soon the vesicle of Purkinje is no longer discoverable. Meanwhile the canal whereby its cavity communicates with the central mass becomes elongated; so that its union with the common substance of the polypary is not destroyed even when the "female polyp" has burst through the external membrane and the thin operculum of the ovarian capsule in which it was formed.
(234). When the "female polyp" has thus escaped from the ovarian vesicle of the common polypary, it has the appearance of a globular transparent capsule attached by a short pedicle to the operculum through which it has made its way, the orifice whereby it escaped having closed around its stem. The tentacles are twelve in number; and from the circle surrounding their base, four canals may be traced, descending in the substance of the globular body to terminate in the pedicle or sacculus that occupies the lower part of the (Medusiform) capsule.
(235). On the rupture of the external membrane of the ovum enclosed in the "female polyp," the young animal escapes, under a form not at all resembling that of the parent animal.
(236). It presents at this period the appearance of a little worm, of an elliptical shape, slightly flattened. Its entire surface is thickly covered with vibratile cilia, by the agency of which it moves about even while still imprisoned in the body of its mother, from which it subsequently makes its escape through the oral orifice. Generally each "female" gives birth to two embryos, occasionally to three.
(237). No sooner has the young larva got free than it begins to swim about, by means of its cilia, with a uniform gliding motion: sometimes it turns round incessantly upon its axis, either horizontally or in a vertical direction, at the same time varying its shape from that of an egg to that of a pear. It is of a whitish colour, but still sufficiently transparent under the microscope to show that it contains a cavity filled with a coloured fluid, and composed of two membranes, whereof one, the outer, is transparent as glass, the internal slightly opake.
(238). Repeated observations render it improbable that in this condition the little embryo is nourished by means of a mouth.
(239). After swimming about for some time in the above condition, the young creature fixes itself to some foreign body, such as a fucus, or other marine production; and then its form begins to be entirely changed, and it is converted into a flat, circular disk, around which the cilia, now quiescent, form a circular transparent fringe. In the centre of its internal cavity an opake round spot makes its appearance, the size of which is about a fifth part of that of the whole body, composed of a mass of granules placed concentrically, and occupying the situation whence the stem of the nascent polypary is to be developed. At this point the external membrane becomes slightly thickened, and, as it were, furrowed with vessels proceeding from the internal cavity. From the opake central spot arises a hemispherical protuberance; and at the same time the central cavity loses its semicircular form and becomes divided into four or five irregular lobes, which subsequently become the horizontal supports of the fixod polypary.
(240). Already the whole expansion is covered with a horny layer; but this only becomes distinctly recognizable at a more advanced stage of growth.
(241). The trunk continues to rise vertically upwards, and ultimately produces at its summit a solitary cell, in which a "male" (nutritive) polyp is gradually developed; and then, as growth advances, secondary ramifications are developed, after the pattern peculiar to the species.
(242). Another important fact in connexion with the history of the Hydrozoa is, that in the compound species there exist male branches as well as female branches upon the same polypary, the latter producing ovigerous vesicles, whilst in the former the ova are replaced by seminal capsules; these almost precisely resemble the "female capsules" (Beroeform gemmae) figured by Loven, and are, in like manner, surmounted by a circle of tentacula.
The polypary or common integument of these zoophytes is composed of a semigelatinous horny substance (fig. 47.) The older stems assume a dark-brown colour and a consistence resembling that of horn. The young branches, on the contrary, and more particularly the polyp-cells, are thin and perfectly diaphanous. The polypary always exhibits a principal trunk, from which the different branches proceed, every one of the latter being terminated by a bell-shaped cell.