_m_ (?) groove above dorsal organ; _Ph._ dorsal organ; _st._ stomodæum (?); _s._ ciliated disc at aboral end of body.] 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. Cyphonautes (fig. 133) forms at certain seasons of the year one of the commonest captures in the surface net. It was originally described by Ehrenberg, but the important discovery of its true nature as the larva of Membranipora (the common species _C. compressus_ is the larva of _Mem. pilosa_), a genus of the chilostomatous Polyzoa, was made by Schneider (No. 322). The younger stages of the larva have not been worked out, but from a comparison with the last described larva it is easy to make out the general relationship of the parts. The larva has a triangular form with an aboral apex, corresponding with the summit of the dome of the Flustrella larva, and an oral base. It is enclosed in a bivalve shell, the two valves of which meet along the two sides, but are separate along the base. At the apex an opening is left between the two valves, through which a ciliated disc (_f.g_) of the same character and nature as that of previous larvæ can be protruded. [FIG. 133. CYPHONAUTES (LARVA OF MEMBRANIPORA). (After Hatschek.) _m._ mouth; _a´_ anus; _f.g._ ciliated disc; _x._ problematical body (probably a bud).]