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