Appendix to Ctenophora
Hydroctena salenskii has recently been discovered by Dawydoff[[435]] floating with the Plankton off the island Saparua in the Malay Archipelago. It is claimed to be a connecting link between the Ctenophora and the Medusae of the Hydrozoa.
In external features it is like one of the Narcomedusae, having a transparent jelly-like bell with a wide bell-mouth guarded by a velum (Fig. 184, V). There are only two simple but solid tentacles (t), provided with tentacle-sheaths, but inserted on opposite sides of the bell—not on the margin, but, as in the Ctenophore, at a level not far removed from the aboral pole. At the aboral pole there is a minute pore surrounded by a high ciliated epithelium bearing an orange pigment. This leads into a short blind canal, which terminates in an ampulla bearing two statoliths supported by elastic processes from the ampullar epithelium.
The sub-umbrellar cavity extends for a distance of about one-half the height of the bell. The mouth (M), which opens into this cavity, leads into a wide cavity that gives off a short blind canal to the side of each tentacular sheath, and a straight tube that leads straight to the statocyst, where it also ends blindly. There are no radial canals and no ring canal at the margin of the umbrella. There are also no ctenophoral plates. In the absence of any information concerning the position of the genital glands, the character of the epithelium of the tentacles and the development, we are not justified in regarding Hydroctena either as a Ctenophore or as a connecting link between the Ctenophora and the Hydromedusae. It may be regarded simply as a Craspedote Medusa, probably related to the Narcomedusae, with a remarkable aberrant aboral sense-organ.
Fig. 184.—Hydroctena salenskii. ab, Aboral organ; M, manubrium; t, tentacle; V, velum. (After Dawydoff.)
ECHINODERMATA
BY
E. W. MacBRIDE, M.A., FRS.
Formerly Fellow of St. John's College
Professor of Zoology in McGill University, Montreal.