CHAPTER VIII.

ROTIFERA.

For many reasons a complete knowledge of the ontogeny of the Rotifera is desirable. They constitute a group which retain in the trochal disc an organ common to the embryos of many other groups, but which in most other instances is lost in the adult state. In the character of the excretory organs they exhibit affinities with the Platyelminthes, while in other respects they possibly approach the Arthropoda (_e.g._ Pedalion ?). The interesting _Trochosphæra æquatorialis_ of Semper closely resembles a monotrochal polychætous larva.

Up to the present time our embryological knowledge is mainly confined to a series of observations by Salensky on _Brachionus urceolaris_, and to scattered statements on other larval forms by Huxley, etc.

In many cases Rotifers lay summer and winter eggs of a different character. The former are always provided with a thin membrane, and frequently undergo development within the oviduct. They are hatched in the autumn. The winter eggs are always provided with a thick shell.

The summer eggs are of two kinds, viz. smaller eggs which become males, and larger, females. On the authority of Cohn (No. 232) they are believed to develop parthenogenetically. Males are not found in summer, and only seem to be produced from the summer eggs. Cohn's observations, especially on _Conochilus volvox_, are however not quite satisfactory. Huxley (No. 234) came to the conclusion that the winter eggs of Lacinularia developed without previous fertilization.

The following are the more important results of Salensky's observations (No. 236) on _Brachionus urceolaris_.

The ovum is attached by a short stalk to the hind end of the body of the female, in which position it undergoes its development. It will be convenient to treat separately the development of the female and male, and to commence with the former. The female ovum divides into two unequal spheres, of which the smaller in the subsequent stages segments more rapidly than the larger. The segmentation ends with the formation of an epibolic gastrula. The solid inner mass of cells derived from the larger sphere constitutes the hypoblast, and is more granular than the epiblast. The evolution of the embryo commences with the formation of a depression on the ventral surface, at the bottom of which the stomodæum is formed by an invagination. At the hinder part of the depression there rises up a rounded protuberance which eventually becomes the caudal appendage or foot. Immediately behind the mouth is formed an underlip.

On the sides of the ventral depression are two ridges which form the lateral boundaries of the trochal disc. They appear to unite with the under lip.

In a later stage the anterior part of the body becomes marked off from the posterior as a præoral lobe, and the hypoblast is at the same time confined to the posterior part. The supra-oesophageal ganglion is early formed as an epiblastic thickening on the dorsal side of the præoral lobe.

The first cilia to appear arise at the apex of the præoral lobe. At a later period the lateral ridges of the trochal disc meet dorsally and so enclose the præoral lobe. They then become coated by a ring of cilia, to which a second ring, completing the double ring of the adult, is added later.

[FIG. 100. EMBRYO OF BRACHIONUS URCEOLARIS SHORTLY BEFORE IT IS HATCHED. (After Salensky.)

_m._ mouth; _ms._ masticatory apparatus; _me._ mesenteron; _an._ anus; _ld._ lateral gland; _ov._ ovary; _t._ tail, _i. e._ foot; _tr._ trochal disc; _sg._ supra-oesophageal ganglion.]

In the trunk an indication of a division into two segments makes its appearance shortly after the development of the præoral lobe. Before this period the proctodæum is established as a shallow pit immediately behind the insertion of the foot. The latter structure soon becomes pointed and forked (fig. 100, _t_).

The complete establishment of the alimentary canal occurs late. The stomodæum (fig. 100) gives rise to the mouth (_m_), oesophagus and masticatory apparatus (_ms_). The mesenteron is formed from the median part of the hypoblast; the lateral parts of which appear to give rise to the great lateral glandular structures (_ld_) which open into the stomach, and to the ovaries (?) (_ov_) etc. The proctodæum becomes the cloaca and anus (_an_). The origin of the mesoblast is not certainly known. The shell is formed before the larva is hatched--an occurrence which does not take place till the larva closely resembles the adult.

The early developmental stages of the male are closely similar to those of the female; and the chief difference between the two appears to consist in the development of the male being arrested at a certain point.

The larvæ of Lacinularia (Huxley, No. 234) are provided with a præoral circlet of cilia containing two eye-spots[98], and a peri-anal patch of cilia. They closely resemble some telotrochal polychætous larvæ.

[98] In Leydig's figure of the larva, _Zeit. f. wiss. Zool._ Vol. III. 1851, the eye-spots lie just outside the ciliated ring.

Salensky has compared the larva of Brachionus to that of a cephalophorous Mollusc, more especially to the larva of Calyptræa on which he has made important observations. The præoral lobe, with the ciliated band, no doubt admits of a comparison with the velum of the larva of Molluscs; but it does so equally, as was first pointed out by Huxley, with the ciliated præoral lobe of the larvæ of many Vermes. It further deserves to be noted that the trochal disc of a Rotifer differs from the velum of a Mollusc in that the eyes and ganglia are placed dorsally to it, and not, as in the velum of a Mollusc, within it. The larva of Lacinularia appears to be an exception to this, since two eye-spots are stated to lie within the circlet of cilia. More important in the comparison is the so-called foot (tail), which arises in the embryo as a prominence between the mouth and anus, and in this respect exactly corresponds with the Molluscan foot.

If Salensky's comparison is correct, and there is something to be said for it, the foot or tail of Rotifers is not a post-anal portion of the trunk, but a ventral appendage, and the segmentation which it frequently exhibits is not to be compared with a true segmentation of the trunk. If the Rotifers, as seems not impossible, exhibit crustacean affinities, the 'foot' may perhaps be best compared with the peculiar ventral spine of the Nauplius larva of _Lepas fascicularis_ (vide Chapter on Crustacea) which in the arrangement of its spines and other points also exhibits a kind of segmentation.

BIBLIOGRAPHY.

(232) F. Cohn. "Ueb. d. Fortpflanzung von Räderthiere." _Zeit. f. wiss. Zool._ Vol. VII. 1856.

(233) F. Cohn. "Bemerkungen ü. Räderthiere." _Zeit. f. wiss. Zool._ Vol. IX. 1858, and Vol. XII. 1862.

(234) T. H. Huxley. "Lacinularia socialis." _Trans. of the Microscopical Society_, 1853.

(235) Fr. Leydig. "Ueb. d. Bau u. d. systematische Stellung d. Räderthiere." _Zeit. f. wiss. Zool._ Vol. VI. 1854.

(236) W. Salensky. "Beit. z. Entwick. von Brachionus urceolaris." _Zeit. f. wiss. Zool._ Vol. XXII. 1872.

(237) C. Semper. "Zoologische Aphorismen. Trochosphæra æquatorialis." _Zeit. f. wiss. Zool._ Vol. XXII. 1872.