Page:Popular Science Monthly Volume 33.djvu/770

750 and the adult jelly-fish lives only long enough to effect the wide distribution of the eggs, and the establishment of new colonies of larvæ.

Something very similar to this has occurred in a few insects. The caterpillar stage of most butterflies is simply a preparatory step directed toward an end, the production of the perfect insect; but the bagworm is a butterfly in which the larval life is most important, for, while the caterpillar lives long, the female insect never escapes from her cocoon, but after her final transformation lays her eggs within it and dies, while the male lives only long enough to find and fertilize the female, and then dies also.

In the case of the hydroids, the power of budding, a power which is almost absent in insects, enables the larval life to assume a degree of importance which it could not have if the larva remained simple, for it has rendered division of labor possible, and has produced polymorphic communities, most of the members of which are out of the line of succession. The gradual reduction of the adult life is also facilitated by the process of budding, as this effects a great increase in the number of adults which come from each egg, and thus secures the sexual reproduction of the species, notwithstanding the shortening of the life of each adult.

The shells of hermit-crabs often carry colonies of another hydroid, which is so similar to Dysmorphosa that a drawing of one will answer for the other. They are almost exactly alike, and it is only after careful examination that any difference between them is discovered; but, inconspicuous as the difference is, it is highly important, for in the second form, Hydractinia, the adult locomotor jelly-fish stage has been completely lost, and the whole life of the species has become centered in the larvæ. The blastostyles produce buds, which acquire some rudimentary traces of the organization of jelly-fish, but they never become free or complete their development. While still on the blastostyles, they produce eggs or spermatozoa, and having thus accomplished their purpose and secured the perpetuation of their race, they die. The life of Hydractinia is shown in Diagram VII:

This is by no means the end of the story, for the many species of hydroids without any jelly-fish stage present all stages in the gradual simplification of the sessile medusa buds, until at last all traces of the structure of the jelly-fish disappear, and they are