Flustrella and Cyphonautes. The next group of larval forms is that of which Cyphonautes is the best known type. The larvæ composing it at first sight appear to have but little in common with the larvæ hitherto described. The researches of Barrois (No. [298]) and Metschnikoff (No. [314]), (but especially those of the former on the early stages of Flustrella hispida, the larva of which is very similar in form to Cyphonautes, though without so great a complexity of organisation), have given a satisfactory basis for a general comparison of Cyphonautes with other ectoproctous larvæ.
The segmentation and early stages of the embryo of Flustrella resemble closely those of Alcyonidium. A projecting ring of large cells is formed, dividing the larva into oral and aboral parts. The oral part soon however becomes very small as compared with the aboral, and becomes vertically flattened so as to be nearly on a level with the ring of large cells. In the next stage the flattening becomes completed; and the ring of large cells surrounds, like the vestibule of the Entoprocta, a flat oral disc. The aboral side is dome-shaped, and forms the greater part of the embryo.
Fig. 132. Advanced larva of Flustrella hispida.
(After Barrois.)
m (?) groove above dorsal organ; Ph. dorsal organ; st. stomodæum (?); s. ciliated disc at aboral end of body.
In the next stage a small disc—the ciliated disc—is formed in the middle of the aboral dome. The larva becomes laterally compressed. The ring of large cells which now constitute the edge of the vestibule is covered, as in the larva of Pedicellina, by cilia, which are specially long in front of the dorsal organ.
In the next stage the ciliated disc ([fig. 132], s.) becomes reduced in size, but surmounted by a ring of cilia round the edge, and a tuft of cilia in the centre. The chief difference between this larva and that of Alcyonidium depends on the small size of the ciliated disc, and the oral position of the ciliated ring in the former. There are intermediate types between these forms of larvaæ.
This stage immediately precedes the liberation of the larva. The free larva differs from that in the ovicell mainly in the possession of a shell formed as a cuticular structure, composed of two valves placed on the two sides of the embryo. The aboral ciliated disc, still more reduced in size, loses its cilia, and becomes enclosed between the two valves of the shell.
The post-embryonic metamorphosis follows, so far as is known, the course already described for the larva of Alcyonidium.