I, Simple disk of Microcodon; 2, bdelloid disk of Rotifer and of most Melicertids showing dorsal gap; 3, disk of Hydatina, with lobed ridges in the groove, bearing vibratile styles (membranelles); 4, disk of Melicerta ringens and M.
There is no difficulty whatever in regarding Hydra as bearing the same relation to the actinula-stage of other Hydromedusae that a Rotifer bears to a trochophore-larva or a fish to a tadpole.
The apical end of the rotifer usually narrows suddenly beyond the curve of the gut and the cloacal aperture to form the foot of pseudopodium which ends in an organ of attachment, a pair of movable toes, each with the opening of a cement-gland (gl) at its tip. Thus for orientation we place the rotifer like the cuttle-fish, head downwards: the ciliated disk is basal or oral, proximal to the rest of the animal, the foot is apical, and the brain and cloacal aperture are anterodorsal.
Leeuwenhoek described Rotifer vulgaris in 1702, and he subsequently described Melicerta ringens and other species.
We homologize the rotifer with the Turbellarian larva (fig.