The development of the Narcomedusae is in the main similar to that of the Trachomedusae, but shows some remarkable features. In Aeginopsis a planula is formed by multipolar immigration. The two ends of the planula become greatly lengthened and give rise to the two primary tentacles of the actinula, of which the mouth arises from one side of the planula. Hence the principal axis of the future medusa corresponds, not to the longitudinal axis of the planula, but to a transverse axis. This is in some degree parallel to the cases described above, in which a planula gives rise to the hydrorhiza, and buds a polyp laterally.
In Cunina and allied genera the actinula, formed in the manner described, has a hypostome of great length, quite disproportionate to the size of the body, and is further endowed with the power of producing buds from a stolon arising from the aboral side of the body. In these species the actinula is parasitic upon another medusa; for instance, Cunoctantha octonaria upon Turritopsis, C. proboscidea upon Liriope or Geryonia. The parasite effects a lodgment in the host either by invading it as a free-swimming planula, or, apparently, in other cases, as a spore-embryo which is captured and swallowed as food by the host. The parasitic actinula is found attached to the proboscis of the medusa; it thrusts its greatly elongated hypostome into the mouth of the medusa and nourishes itself upon the food in the digestive cavity of its host. At the same time it produces buds from an aboral stolon. The buds become medusae by the direct method of budding described above. In some cases the buds do not become detached at once, but the stolon continues to grow and to produce more buds, forming a “bud-spike” (Knospenähre), which consists of the axial stolon bearing medusa-buds in all stages of development. In such cases the original parent-actinula does not itself become a medusa, but remains arrested in development and ultimately dies off, so that a true alternation of generations is brought about. It is in these parasitic forms that we meet with the method of reproduction by sporogony described above.
In other Narcomedusae, e.g. Cunoctantha fowleri Browne, buds are formed from the sub-umbrella on the under side of the stomach pouches, where later the gonads are developed.
Classification.—Three families of Narcomedusae are recognized (see O. Maas [40]):
| After O. Maas, Craspedoten Medusen der Siboga Expedition, by permission of E. S. Brill & Co. |
| Fig. 67.—Solmundella bitentaculata (Quoy and Gaimard). |
1. Cunanthidae.—With broad gastric pouches which are simple, i.e. undivided, and “pernemal,” i.e. correspond in position with the tentacles. Cunina (fig. 66) with more than eight tentacles; Cunoctantha with eight tentacles, four perradial, four interradial.
2. Aeginidae.—Radii a multiple of four, with radial gastric pouches bifurcated or subdivided; the tentacles are implanted in the notch between the two subdivisions of each (primary) gastric pouch, hence the (secondary) gastric pouches appear to be “internemal” in position, i.e. to alternate in position with the tentacles. Aegina, with four tentacles and eight pouches; Aeginura (fig. 25), with eight tentacles and sixteen pouches; Solmundella (fig. 67), with two tentacles and eight pouches; Aeginopsis (fig. 23), with two or four tentacles and sixteen pouches.
3. Solmaridae.—No gastric pouches; the numerous tentacles arise direct from the stomach, into which also the peronial canals open, so that the ring-canal is cut up into separate festoons. Solmaris, Pegantha, Polyxenia, &c. To this family should be referred, probably, the genus Hydroctena, described by C. Dawydov [11a] and regarded by him as intermediate between Hydromedusae and Ctenophora. See O. Maas [35].
Appendix to the Trachylinae.
Of doubtful position, but commonly referred to the Trachylinae, are the two genera of fresh-water medusae, Limnocodium and Limnocnida.