The medusome stage in the life-history of these Hydrozoa is produced by gemmation from the hydrosome, or, in some cases, by gemmation from the medusome as well as from the hydrosome. In many genera and species the medusome is set free as a minute jelly-fish or Medusa, which grows and develops as an independent organism until the time when the sexual cells are ripe, and then apparently it dies. In other Gymnoblastea the medusome either in the female or the male or in both sexes does not become detached from the parent hydrosome, but bears the ripe sexual cells, discharges them into the water, and degenerates without leading an independent life at all. In these cases the principal organs of the medusome are almost or entirely functionless, and they exhibit more or less imperfect development, or they may be so rudimentary that the medusoid characters are no longer obvious. Both the free and the undetached medusomes are gonophores, that is to say, the bearers of the sexual cells, but the former were described by Allman as the "phanerocodonic" gonophores, i.e. "with manifest bells," and the latter as the "adelocodonic" gonophores. The gonophores may arise either from an ordinary zooid of the colony (Syncoryne), from a specially modified zooid—the blastostyle—as in Hydractinia, or from the hydrorhiza as in certain species of Perigonimus. The free-swimming Medusa may itself produce Medusae by gemmation from the manubrium (Sarsia, Lizzia, Rathkea, and others), from the base of the tentacles (Sarsia, Corymorpha, Hybocodon), or from the margin of the umbrella (Eleutheria).
The free-swimming Medusae or phanerocodonic gonophores of the Gymnoblastea are usually of small size (1 or 2 mm. in diameter) when first liberated, and rarely attain a great size even when fully mature. They consist of a circular, bell-shaped or flattened disc—the umbrella—provided at its margin with a few or numerous tentacles, and a tubular manubrium bearing the mouth depending from the exact centre of the under (oral) side of the umbrella (Fig. 132, A). The mouth leads into a shallow digestive cavity, from which radial canals pass through the substance of the umbrella to join a ring-canal at the margin (Fig. 131).
The sense-organs of the Medusae of the Gymnoblastea are in the form of pigment-spots or very simple eyes (ocelli), situated at the bases of the tentacles. The orifice of the umbrella is guarded by a thin shelf or membrane, as in the Calyptoblastea, called the velum. The sexual cells are borne by the manubrium (Figs. 131 and 132, A).
There are many modifications observed in the different genera as regards the number of tentacles, the number and character of the radial canals, the minute structure of the sense-organs, and some other characters, but they agree in having a velum, ocellar sense-organs, and manubrial sexual organs. The tentacles are rudimentary in Amalthea; in Corymorpha there is only one tentacle; in Perigonimus there are two; and in Bougainvillia they are numerous; but the usual number is four or six. The radial canals are usually simple and four in number, but there are six in Lar sabellarum, which branch twice or three times before reaching the margin of the umbrella (Fig. 132, B).
Fig. 131.—Medusa of Cladonema, from the Bahamas, showing peculiar tentacular processes on the tentacles, the ocelli at the base of the tentacles, the swellings on the manubrium that mark the position of the gonads, and the radial and ring-canals of the umbrella. (After Perkins.)
There can be no doubt that the Medusae of many Gymnoblastea undergo several important changes in their anatomical features during the period of the ripening of the sexual cells. Thus in Lar sabellarum the six radial canals are simple in the first stage of development (A); but in the second stage (B) each radial canal bifurcates before reaching the margin, and in the adult stage shows a double bifurcation. The life-history has, however, been worked out in very few of the Anthomedusae, and there can be little doubt that as our knowledge grows several forms which are now known as distinct species will be found to be different stages of growth of the same species.
Fig. 132.—Two stages in the development of the Medusa of Lar sabellarum (Willsia stellata). A, first stage with six canals without branches; B, third stage with six canals each with two lateral branches. The developing gonads may be seen on the manubrium in A. (After Browne.)
The movements of the Medusae are well described by Allman[[301]] in his account of Cladonema radiatum:—"It is impossible to grow tired of watching this beautiful medusa; sometimes while dashing through the water with vigorous diastole and systole, it will all at once attach its grapples to the side of the vessel, and become suddenly arrested in its career, and then after a period of repose, during which its branched tentacles are thrown back over its umbrella and extended into long filaments which float, like some microscopic sea-weed in the water, it will once more free itself from its moorings and start off with renewed energy." The Medusa of Clavatella, "in its movements and mode of life, presents a marked contrast to the medusiform zooid of other Hydrozoa. The latter is active and mercurial, dancing gaily through the water by means of the vigorous strokes of its crystalline swimming-bell. The former strides leisurely along, or, using the adhesive discs as hands, climbs amongst the branches of the weed. In the latter stage of its existence it becomes stationary, fixing itself by means of its suckers; and thus it remains, the capitate arms standing out rigidly, like the rays of a starfish, until the embryos are ready to escape."[[302]]