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.
(1153). A vibratile body is contained in each of the caecal branches, and there is likewise one on each side in the transverse connecting branch. Two more are contained in each lateral main trunk, one opposite the pancreatic sacs, and one lower down, making in all five on each side. Each of these vibrating bodies is a long cilium (1/1000 of an inch), attached by one extremity to the side of the vessel, and by the other vibrating with a quick undulatory motion in its cavity, giving rise, as Siebold remarks, to an appearance singularly like that of a flickering flame.
(1154). The last subject that we have to consider relative to the internal economy of the Rotifera is the conformation of their generative apparatus, which here assumes a considerable perfection of development. The reproductive system is composed apparently of two distinct parts; the one subservient to the formation of the ova, the other destined either to furnish some secretion essential to the completion of the egg, or, as has been surmised, secreting a fertilizing fluid by which the impregnation of the ova is effected prior to their escape from the body.
(1155). In Melicerta ringens, as we learn from Professor Williamson's admirable memoir, the ovary is a hollow sac, consisting of a very thin pellucid membrane, filled with a viscid granular protoplasm of a light-grey colour, in which may be perceived some twenty or thirty nuclei, each containing a nucleolus in its interior: these seem to be successively selected for development after the following manner: - One of the nuclei situated near the surface of the ovary attracts around it a small portion of the granular protoplasm, which becomes detached from the remaining contents of the ovary. The portion thus specially isolated gradually enlarges, assuming at the same time a darker hue, and the nucleus slightly enlarges, while its central nucleolus appears to become absorbed. When the ovum, thus separated from the ovarian protoplasm, has attained its full size, it becomes invested by a thin shell, which is apparently a secretion from its own surface. The ovum being thus ready for expulsion (fig. 227,1, q), is slowly forced down to the lower part of the ovary, and, sweeping round the inferior border of the lower stomach, passes through the dilated oviduct (fig. 227,1,1) and enters the cloaca, whence, by a sudden contraction, it is expelled.
* Quarterly Journal of Microscopical Science, no. 1. p. 6.
(1156). At this point of development the yelk consists of a single segment; but very soon the central nucleus becomes drawn out and divides into two, this division being followed by a corresponding segmentation of the yelk. The same process is repeated over and over again, until at length the yelk becomes converted into a mass of minute cells. The first trace of further organization which presents itself appears in the form of a few freely-moving cilia; these are developed at two points, one at c a, fig. 231,1, which corresponds with the future head, and the other near the centre of the ovum (b), which is destined to become the cavity of the stomach; shortly after this appearance of cilia, traces of the dental apparatus become recognizable, - this, again, being soon succeeded by the union of the entire mass of yelk-cells, and the formation from them of the various organs of the animal. The cilia now play very freely, especially at the head (a); the creature twists itself about in its shell; and two red spots (c c), regarded by Ehrenberg as organs of vision, appear. The young animal now bursts its shell, and presents the appearance represented in fig. 231, - its whole organization, though obscurely seen, being that of the perfect animal, and not of a larval state.
(1157). The young Melicerta, when first hatched, is free, and swims about actively in the water for a short period, when, attaching itself by its caudal extremity to some foreign object, it proceeds to manufacture for itself a tube for its future residence by means of a most remarkable apparatus appointed for the purpose. This is the appendage called by Professor Williamson the fifth rotatory flap (fig. 227,1, c), and named by Mr. Gosse* the "chin" or "pellet-cup" in which the minute masses whereof the tube is formed are prepared. Into this cup-like organ foreign particles are continually brought, at the will of the animal, by ciliary action, and collected into little pellets, which are deposited as quickly as they are completed in successive rows around the foot, until a tube is formed of sufficient size for the lodgment of the little Rotifer (fig. 231, 2, a, b).
Fig. 231. Young Melicerta.
* Quarterly Journal of Microscopical Science.
(1158). In the remarkable genus Asplanchnia, it has been ascertained by Mr. Dalrymple* that these animals are bisexual, and, moreover, that the male individual is one of the strangest organisms as yet discovered. In the female the sexual apparatus is very completely developed, consisting of an ovary, an ovisac, vaginal canal, and vulva, the whole being so transparent that the development of the embryo throughout all its stages is readily observable, and its progress traced from the time of the formation of the egg to its birth. The eggs are of three kinds: the first is of ordinary structure, wherein the formation of the female embryo is easily witnessed while contained in the body of the parent; but towards the latter end of the season, ova are furnished of a totally different character, which are apparently destined to remain through the winter undeveloped until the following year. In a third description of ovum an embryo may be observed to become developed gradually from a germinal vesicle until it begins to assume a definite shape and independent movement, when we are at once struck with the remarkable peculiarities observable in its form, size, and organization; and this is the male, which will require special description.