an opening to the exterior is formed. The central space now becomes the stomach, d, in Figs. 4 and 5, and the opening becomes the month, e, in Figs. 4 and 5. Soon after the mouth is formed a circlet of tentacles is developed around it, and the larva becomes a hydra, but a hydra adapted to a floating life rather than a fixed life, as shown in Fig. G.
It is now able to capture and digest food and to lead an independent life, and it grows rapidly, although it has as yet no locomotor organs, and drifts at the mercy of the waves.
Soon the stomach becomes flattened and the mouth pushes in toward the center of the sphere, carrying with it the surface layer of cells, and thus giving rise to a concave, hollow bell-cavity opening to the exterior, as shown in Fig. 7, in which d is the flattened stomach, e the mouth, and f the cavity of the swim-ball.
As the mouth is pushed in, the tentacles are left behind and remain on the edge of the bell in the position which they occupy in the adult. The animal grows rapidly, the tentacles lengthen, and, after some slight changes which do not now concern us, the larva becomes converted into an adult, like the one shown in Fig. 2, and again reproduces its kind by fertilized eggs.
The life-history of Liriope is simple and direct. There is a metamorphosis, and the animal passes through a planula stage, a hydra stage, and a medusa stage; but its identity is never lost, and the larva is the same individual as the adult, as is shown in the following diagram:
II. Liriope.Egg Planula free Hydra Medusa < eggs.
Although most of the text-books state that the direct history of Liriope has been produced by the gradual simplification of a complicated life-history like that of Dysmorphosa, there is no evidence whatever that this is the case, and the fact that the development of Liriope resembles that of all ordinary animals is in itself an indication that its simplicity is not secondary but primitive. This view is rendered still more certain by the study of other jelly-fishes which exhibit successive steps in the process of complication.
