[96] Annal. d. Scien. Nat., 6th Series, Vol. VI. 1877.
[97] Beobachtungen üb. Anat. u. Entwick. Wirbell. Thiere. Leipzig, 1863.
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.