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.
(257). The marginal tubercles situated around the disk (fig. 48, e) now become sensibly elongated, and the whole embryo presents the appearance of a minute star-fish, the elongate tubercles representing the rays.
(258). The nuclei in the interior of the marginal (tentacular) tubercles next become elongated, together with their containing cells, rendering the rays hollow in the centre; and soon new cells are discoverable in their interior, the number of which is limited, and probably the same, in all the rays. The appearance of these secondary cells causes a rapid increase in the length of the tentacula, and their remains give rise to numerous septa, producing an appearance somewhat analogous to that of the transverse striae of muscular fibre.
(259). The embryo animal, be it observed, is as yet still contained in the ovary of the polyp; but it is already capable of distinct and continual movements, perceptible through the walls of the ovarian vesicle.
(260). A tubercle (fig. 48, f, b) is developed from the centre of the under surface of the disk, which represents the central pedicle met with under various forms among the Medusae, and which may be called the proboscidiform appendage. This organ can contract or extend itself in all directions, constantly changing its form, and resembling in no slight degree the body of a Hydra. At an early period an opening is perceptible at the extremity of this appendage, which evidently represents a mouth, being in direct communication with the vitelline cavity.
(261). The vitelline or digestive cavity, now that there is a mouth, increases in size in proportion to the growth of the embryo, but still preserves its sac-like shape. It is partially filled up with irregular granules, which become perceptible at a very early period - at first colourless, but gradually becoming of a yellowish tinge. Towards the close of this period of development the granules seem to be heaped together into a mass, from which all the nutritive part appears to have been extracted. This constitutes the meconium.
(262). In some instances, the nutritious fluid that circulates in the interior of the parent polypary may be seen to penetrate as far as the interior of the vitelline cavity of the embryo, which thus seems to derive its nourishment at the expense of the general community; and when it is taken into consideration that the ova are formed in the common fleshy substance lining the walls of the ovarian vesicle, and that the nutritious fluid is diffused throughout its entire mass, it is easy to understand how, after the |external membrane surrounding the embryo is ruptured, it is enabled to penetrate, by means of the mouth, into the interior of its stomachal cavity.
(263). Mention has been made, in the above description (§ 256), of cells which give origin to organs of sensation, and which make their appearance at a very early period. These present the same appearance as the eyes and the ears of the lower mollusca and other inferior animals, and moreover present a similar organization, being composed of two spherical vesicles enclosed one within the other. That the young polyp possesses these organs of relation with the external world is undeniable, although no traces of them remain when the animal has acquired its full development; but what is still more surprising, according to the researches of Van Beneden, coexistent with these instruments of sense, there are perceptible a muscular system and an apparatus of nerves and nervous ganglia which, like the preceding, are only of a temporary character. While the young polyp is still enclosed in its cell, two bands, apparently composed of muscular fibre (fig. 48, p, d), make their appearance; these run from one margin of the disk to the opposite edge, crossing each other at right angles, in the centre, so as to present a cruciform arrangement. These bands are quite isolated, and their muscular fibres distinct and transparent. By their action the margins of the disk are approximated, enabling these little animals to imitate the movements so characteristic of the Medusae.
(264). Situated upon the course of the bands above described, close to the edge of the vitelline sac, are little rounded bodies (fig. 48, r, e e), presenting an irregular and slightly tuberculated surface, considered by Van Beneden to be nervous ganglia. These little bodies are four in number. No filaments of intercommunication or nervous cords have as yet been detected even proceeding to the organs of sensation, but the ganglia seem to be adherent to the muscular bands apparently by the intermedium of nerves.
(265). It may appear a little rash, says the eminent observer to whom science is so much indebted for these researches, to speak of muscles, nerves, and organs of sensation in the embryo of a polyp, which at a later period presents no traces of the existence of such apparatus; nevertheless the polyp, during its free state, must necessarily require such instruments of sense, to enable it to select a situation adapted to the reception of the new colony to which it gives birth: when once it has made choice of a fit locality, such organs become as useless as they were formerly needful, seeing that all the functions of life are restricted to those of alimentation and reproduction.
(266). The young Campanulariai, arrived at this stage of development, abandon the ovarian vesicle of the parent polypary and swim freely about in the surrounding medium, exactly resembling so many young Medusae (fig. 48, g).