Page:Popular Science Monthly Volume 33.djvu/767

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V. Turritopsis. — Egg = Planula = Root x \begin{matrix}\Bigg\{\end{matrix} Hydra x \begin{matrix}\big\{\end{matrix} Medusa < eggs.
Medusa < eggs.
Hydra x \begin{matrix}\big\{\end{matrix} Medusa < eggs.
Medusa < eggs.
Hydra x \begin{matrix}\big\{\end{matrix} Medusa < eggs.
Medusa < eggs.

The life-history of Turritopsis is therefore like this, and the chain which connects the egg with the adult is broken three times, for the root, which is directly derived from the egg, goes no further, nor do the hydras which bud from the root become jelly-fish, and the latter form still a third set of individuals.

The larval life is long and important; the number of sexual adults produced by each egg is very great indeed, and the life-history is extremely complicated, but each one of the individuals is in the direct line of succession; for, while neither the root nor the hydras ever become converted into any higher form, the root produces hydras, and each one of these produces jelly-fish.

In the next species to be considered, a Eutima which is common on our coast (Fig. 16), another stage of complexity is introduced by the restriction of the power to bud jelly-fish to certain hydras, while others become specialized for nutrition. This specialization has come about gradually, and the various species of living hydroids exhibit all the steps in the process. In some species, as in Turritopsis, all the hydras perform both functions, and are alike in structure;

PSM V33 D767 The planula of eutima.jpg
Fig. 17.

in others, those which are placed at the tips of the branches and are best able to obtain food devote themselves to this purpose and produce no jelly-fish, while these are budded only from those hydras which are near the base of the colony. In some cases the two sets of hydras are alike in structure, but in other species the feeding hydras at the tips of the branches are very large, with capacious stomachs and long tentacles, while the reproductive hydras have small tentacles and mouths. In still other species, as in Eutima, they are true blastostyles, without mouths, and with rudimentary tentacles, and all the work of nutrition is performed by the feeding hydras.

The planula of Eutima is shown in Fig. 17. After a short swimming life, it fastens itself to some solid body, and elongating, be-