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).
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 larvæ. 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 larvæ. The deeper part of the vestibule leads into the mouth (m) and œsophagus; the latter is continued till close to the apex of the larva, there bends upon itself, dilates into a stomach, and is continued parallel to the œsophagus 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 is somewhat doubtful. It was regarded as muscular by Claparède (No. [309]), though this, as shewn by Schneider, is no doubt a mistake. Allman (No. [305]) regards it as hepatic, and Hatschek as a thickening of the epidermis. Close to each of these organs is a small body regarded by Claparède as an accessory muscle. It is placed in the normal position for a Polyzoon ganglion, and may perhaps be therefore regarded as nervous in nature. Allman points out its similarity to a bilobed ganglion, but is not inclined to take this view of it. The constitution of the parts contained in the anterior cavity (x) is somewhat obscure. The most elaborate descriptions of them are given by Schneider and Allman. Lining the cavity is apparently a mass of spherical bodies, connected with which is a tongue-like process provided with long cilia, which can be protruded from the orifice. Internal to this is a striated body. A good figure of the whole structure is given by Schneider.
The general similarity of Cyphonautes to the other larvæ is quite obvious from the above description and figure. In the presence of an anus, a vestibule, and possibly a nervous system, it clearly exhibits a far more complicated organisation than any other Polyzoon larvæ except those of the Entoprocta.
The post-embryonic metamorphosis of Cyphonautes, admirably investigated by Schneider, takes place in the same manner as that of other larvæ, and is accompanied by the degeneration of the larval organs, and the formation of a clear body, which gives rise to the alimentary cavity and lophophore of the fixed polype. The larval shell takes part in the formation of the ectocyst of the polype.
Cyclostomata. We owe to Barrois by far the fullest account of the development of the Cyclostomata, but how far his interpretations are to be trusted is very doubtful. The larvæ differ very considerably from the normal larvæ of the Chilostomata and Ctenostomata; the difference being mainly due to the enormous development of the ciliated disc. Barrois has investigated the larvæ of three genera, Phalangella, Crisia, and Diastopora, and states that they very closely resemble each other. The ovum is extremely minute.
The segmentation, so far as it has been made out, is regular. During the segmentation growth is very rapid, and eventually there is formed a blastosphere many times larger than the original ovum. The blastosphere becomes flattened, and is converted into a gastrula by bending up into a cup-like form. The gastrula opening is stated to remain as the permanent mouth, which has a terminal and central position. A transverse ring-like thickening is formed round the larva, which probably corresponds with the ciliated ring of previous larvæ; and the body of the larva in front of this ring becomes ciliated. The aboral end of the larva becomes thickened, and grows out into an elongated prominence, which probably corresponds to the ciliated disc. The ring before mentioned becomes at the same time more prominent, and forms a cylindrical sheath for the ciliated disc. At the time when the larva becomes liberated from the maternal cell it has the form of a barrel with a slight constriction in the middle separating the oral from the aboral end. At the centre of the oral face is situated the mouth, leading into a wide stomach, while the aboral end is formed of the ciliated disc enclosed in its sheath. The whole surface is now ciliated. No structure equivalent to the dorsal organ or bud is described by Barrois, but in other respects, if the ciliated disc is really equivalent in the two forms, a general comparison on the line indicated above between this larva and the normal larvæ of the Ctenostomata and Chilostomata seems quite possible. The fixation and subsequent development of the larva take place in the normal manner.