Turritopsis, the jelly-fish, which is infested by the Cuinina larvæ, has a life-history which is very similar to the one given in this diagram, with the addition of a slight but highly important

modification.

The planula is shown in the left-hand lower corner of Fig. 2. It soon attaches itself to some solid body and becomes a root, which goes no further, but, as shown in the right-hand lower corner of Fig. 2, soon produces a bud which becomes a feeding hydra. Multiplication by budding now goes on rapidly, in such a way as to