[Illustration: Fig. 79. Melicertum campanula seen from above; _m_ mouth, _o o_ ovaries, _t t_ tentacles. (_Agassiz_.)]
[Illustration: Fig. 80. Melicertum seen in profile; natural size.]
In the Melicertum and Tima we have had the good fortune to trace the process by which the eggs are changed into Hydroid communities. If any one has a curiosity to follow for themselves this singular history of alternate generations, the Melicertum is a good subject for the experiment, as it thrives well in confinement. After keeping a number of them in a large glass jar for a couple of days at the time of spawning, it will be found that the ovaries, which were at first quite full of eggs, are emptied, and that a number of planulæ; are swimming about near the bottom of the vessel. After a day or two the outline of these planulæ, spherical at first, becomes pear-shaped (see Fig. 81), and presently they attach themselves by the blunt end to the bottom of the jar. (Fig. 82.) Thus their Hydroid life begins; they elongate gradually, the horny sheath is formed around them, tentacles arise on the upper end, short and stunted at first, but tapering rapidly out into fine flexible feelers, the stem branches, and we have a little Hydroid community (Fig. 83), upon which, in the course of the following spring, the reproductive calycles containing the Medusæ buds will be developed, as in the case of the Eucope and Clytia. The Tima passes through exactly the same process, though the shape of the planulæ and the appearance of the young differ from that of the Melicertum, as may be seen in Fig. 78, where a single head of the Tima Hydroid, greatly magnified, is represented. By combining the above observations upon the development of the Hydroids of the Melicertum and Tima with those previously mentioned upon the young Medusa arising from reproductive calycles in the Eucope and Clytia, we get a complete picture of all the changes through which any one of these Hydroid Medusæ passes, from its Hydroid condition to the moment when it enters upon an independent existence as a free Jelly-fish.
[Illustration: Fig. 81. Planula of Melicertum; magnified.]
[Illustration: Fig. 82. Cluster of planulæ just attached to the ground.]