Page:Popular Science Monthly Volume 8.djvu/38

28 The most remarkable fact in regard to these medusæ is, that the immature form shows a higher type, a greater differentiation of organs, than the parent hydroid. The medusa possesses, in common with the parent, a digestive cavity and enidæ; and, in addition to these, an organ at the base of each tentacle, which, if it does not unite within itself the senses of sight and hearing, at least is akin to those organs in the lower invertebrates. They certainly possess distinct bundles of muscles and nerve-ganglia, which are not found in the parent form. When the roving medusa has sown its wild-oats, and comes to settle down into a respectable family hydroid, it loses all these advantages belonging to its wandering life, and becomes in its later form identical with the parent; it returns to the privileges and traditions of its fathers.



The huge Rhizostoma, and the beautiful Chrysaora, common to the English coast, Carpenter tells us, are oceanic medusæ developed from a small hydroid stem. The embryo emerges in the form of a ciliated ovule, resembling some of the infusoria. One end contracts, forms a foot and attaches itself, the other sends out four tubular offshoots, as tentacles, and "the central cells melt down to form the cavity of the stomach." This hydra-like form multiplies in the ordinary way by budding, for an indefinite length of time. After a while, however, a change takes place, the stem shows constrictions, beginning near the distal end, till the whole stem looks like a rouleau of coins; the constrictions deepen, making the stem look like a pile of saucer-shaped bodies; the disks become serrated, and finally the tentacles which belonged to the original medusæ disappear, and new tentacles are formed upon the uppermost disk of the pile. Soon this disk begins to show a sort of convulsive struggle which results in its freeing itself, and swimming away as a medusa; each disk develops in the same way, and in turn separates itself from the parent stem. The original