Four larval forms can be distinguished, viz.
(1) A larval form which with slight modifications is common to all the genera of the Chilostomata (except Membranipora and Flustrella) and of the Ctenostomata.
(2) A bivalved larva of Membranipora known as Cyphonautes, the true nature of which was first recognized by Schneider (No. [322]), and the closely allied larva of Flustrella.
(3) The typical Cyclostomatous larva, for the first full description of which we are indebted to Barrois (No. [298]).
(4) The larva of the Phylactolæmata.
Chilostomata and Ctenostomata. As an example of the first type of larvæ, Alcyonidium mytili, one of the Ctenostomata, may be conveniently selected for description, as having been more completely worked out by Barrois than perhaps any other form. The segmentation commences in the normal manner by the appearance of two vertical furrows followed by an equatorial furrow, which divide the ovum into eight equal spheres. The stage with eight spheres is followed, according to Barrois, by one with sixteen, formed in a remarkable manner by the simultaneous appearance of two vertical furrows, both parallel to one of the original vertical furrows, so that the segmentation spheres at this stage are arranged in two layers of eight each. In the next stage segmentation takes place along two fresh vertical planes, similar to those of the last stage, but at right angles to them, and therefore parallel to the second of the two primitive vertical furrows. At the close of this stage there are thirty-two cells arranged in two layers of sixteen each; and when viewed from the surface each of these layers presents a regularly symmetrical pattern. Up to the stage with sixteen cells the two poles of the egg, separated by the primitive equatorial plane of segmentation, remain equal, but during the stage with thirty-two cells a peculiar change takes place in the character of the cells at the two poles. At the one pole, which will be spoken of as the oral pole, the four central cells become much larger than the twelve peripheral cells.
The stages immediately following are still involved in much obscurity, and have been described very differently by Barrois in his original memoir (No. [298]), and in a subsequent note (No. [307])[122]. In the latter he states that the four large cells of the oral face become enclosed by the division and growth of the twelve peripheral cells. They are thus carried into the interior of the ovum; and there divide into a central vitelline mass—the hypoblast—and a peripheral mesoblastic layer.
The eight peripheral cells of the aboral pole divide vertically, and, owing to the eight central cells at the aboral pole dividing transversely so as to form a protuberance on the aboral surface, they constitute a transverse ring of large cells round the ovum, which become ciliated and constitute the main ciliated band of the embryo, corresponding to the ciliated band at the edge of the vestibule of the entoproctous larvæ. They divide the embryo into an aboral and an oral region. The central part of the aboral projection forms a structure which I shall speak of as the ciliated disc. It probably corresponds with the ciliated disc in the Entoprocta. An invagination is next formed on the oral surface, which gives rise to a sack opening to the exterior ([fig. 131], st.). This was originally held by Barrois to be the stomach; but Barrois now prefers to call it ‘the internal sack.’ To my mind it is probably the stomodæum. The embryo has become in the meantime laterally compressed, and, at what I shall call the anterior end of the oral disc, a structure makes its appearance ([fig. 131], m), which is probably homologous with the dorsal organ of the larva of Pedicellina and may go by the same name. It was originally interpreted by Barrois as the pharynx[123].
