POLYZOA. 



251 



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 origi- 

 nally 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 larvas can be protruded. 



The oral side or base is girthed by a somewhat sinuous ciliated 

 edge, which is continued round the anterior and posterior extremities 

 of the oral disc. It is no doubt equivalent to the ciliated ring of 

 other larva?. Two openings are present on the oral face, both 

 enclosed in a special lobe of the ciliated ring. The larger of these 

 leads into a depression, 

 which may be called the 

 vestibule; and is situated 

 on the posterior side of 

 the oral surface. The 

 smaller of the two, on the 

 anterior side, leads into a 

 cavity which is apparently 

 (Hatschek) equivalent to 

 the rudimentary bud or 

 dorsal organ of other larvae. 

 The deeper part of the 

 vestibule leads into the 

 mouth (m] and oesopha- 

 gus; the latter is continued 

 till close to the apex of 

 the larva, there bends up- 

 on itself, dilates into a 

 stomach, and is continued 



FIG. 133. CYPHONAUTES (LARVA OF MEMBEANI 

 POKA). (After Hatschek.) 



m. mouth; a', anus; f.g. ciliated disc; 

 problematical body (probably a bud). 



x. 



parallel to the oesophagus as the rectum which opens by an 

 anus (a!) at the posterior end of the vestibule. A peculiar paired 

 organ is situated on each side nearly above the stomach. Its nature 



