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.
(293). Professor Forbes, in his admirable monograph upon the British Naked-eyed Medusas, not only confirms the above important observation of the Norwegian naturalist, but describes four different modes of gemmiparous reproduction as occurring in that group of the Acalepha?. 1st, gemmation from the ovaries, as noticed by Sars in Thaumantias multicirrata; 2nd, a mode of gemmation from the pedunculated stomach, which he calls subsymmetrical, because in this case four gemma) are symmetrically arranged round the peduncle, one of which is constantly in a more advanced condition of development than the other three; 3rd, gemmation irregularly from the walls of a tubular proboscis - in which there is no order of development with respect to position, individuals springing indifferently from various parts of the peduncle (fig. 52); and a fourth mode, which is very remarkable, in a new British species named Sarsia prolifera, in which the buds are produced at the bases or tubercles of the four marginal tentacles, and hang from them in bunches like grapes. The degree of development is not equal in all four bunches, and in each case buds are seen in very various stages of advancement, from embryo wart-like sproutings to miniature Medusas, simulating, in their essential characters, the parent animal.
(294). We have already seen, at the close of the last chapter, that the progeny of the Hydriform polyps, during one phasis of their development, were strictly medusoid in their form and organization; and in like manner it is now incontrovertibly established that the Acalephse are, at a certain stage of their growth, to all intents and purposes Hydriform polyps, as will be immediately evident.
Fig. 52. Lizzia octopunctata.
* Fauna Norvegica.
(295). The Acalephs are now universally admitted to be bisexual; and the generative apparatus in both sexes is invariably found to be more or less intimately in relation with the alimentary canal: that is to say, as in the case of the polyps, the reproductive organs are appendages derived from the internal or nutritive system of the body. In both the males and females of the great majority of genera, the testes of the former and the ovaria of the latter are similarly disposed, and present externally precisely the same structure, consisting of duplica-tures of a delicate membrane, between which, in the case of the female, ova are developed in great numbers, generally of a rich orange or purple colour, so as to be conspicuously visible. In the male Acaleph, instead of ova, the generative membrane secretes a vivifying fluid, rich in spermatozoa, and consequently easily recognizable under the microscope.
(296). In Cyanea aurita the generative apparatus of the female consists of four membranous ovaria, easily recognizable on account of their bright colour, which is usually violet, or deep yellow. Their form is generally semicircular (fig. 53), and they are lodged in as many distinct cavities, situated in the immediate vicinity of the central stomachs. Each of these cavities communicates freely with the external element by means of a large round or oval orifice, furnished internally with tentacula having suckers at their extremities (fig. 53, d.) The four semicircular ovaries are each composed of a simple contorted tube (fig. 53, a, b): when full of eggs, its colour is a beautiful violet; but when empty, or when the ova are only partially developed, a yellowish brown.
(297). The ova are not retained in the ovaria during the whole time of their development, neither do they remain in the ovigerons cavity, but escape from the orifice of the latter into the surrounding water, from whence they are again taken up by the tentacula and by the two laminae of the arms, and become lodged in little pouches formed by the laminated margin, in which they undergo further metamorphosis and development. These ovigerous pouches are only met with at certain seasons, disappearing when their functions are accomplished.
Fig. 53. 1. Ovarium of Cyanea aurita.
2. Ciliated embryo after its escape.
(298). The eggs are of a rounded form and covered with a smooth, thin, membranous envelope whilst they remain in the ovary; internally they are filled with a finely-granular mass of a violet hue.
(299). The ova contained in the arm-sacculi are destitute of any shell, and present themselves under three distinct forms, which are very remarkable. Some resemble blackberries, and are of a pale violet hue; others have the shape of minute thick disks, likewise violet, resembling little Medusae deprived of arms and without any nutrient canals; lastly, others are met with (and these latter are the most numerous) which have a cylindrical shape, truncated at both ends, and of a brownish-yellow colour. The two last-mentioned forms are densely covered with cilia, and swim about with facility; the largest among them measure about 1/8th of a line in diameter.
(300). Subsequently the ciliated embryos, escaping from their confinement, detach themselves from the cradles wherein they have been nursed up to this period, and swim freely about in the surrounding water until ripe for a further change in their economy; they then settle down upon some foreign object, such as a piece of sea-weed, to which they attach themselves by one extremity (fig. 54, 3), assuming the appearance of a contracted Hydra, but, as yet, unprovided either with mouth or tentacula; gradually, however, an oral aperture and stomachal cavity, surrounded by tentacular organs, become apparent; and as these progressively increase in number (fig. 54, 4, 5, 6, 11), the little creature assumes completely the polyp form, and, what is still more wonderful, acquires in this early and, as it might be called, larva-condition of its existence the power of multiplying itself under the same shape, apparently ad infinitum.