Page:Popular Science Monthly Volume 33.djvu/760

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N the first part of this article I described the life-histories of two hydroids: one, Liriope, in which each egg gives rise to only one jelly-fish, which is solitary and free at all stages of its existence, and without any power to multiply asexually; and a second species, Dysmorphosa, in which there is no limit to the number of adults to which a single egg may give rise, and in which the life-history is a complicated alternation of generations, with a sessile polymorphic hydroid stage from which the sexual jelly-fishes are produced by budding.

I shall now briefly sketch the more prominent features in the history of the process of specialization which has gradually evolved a complicated life-cycle like that of Dysmorphosa from one as simple and direct as that of Liriope, The parasitic jelly-fishes are peculiarly instructive in this connection. The genus Cunina includes a number of species which, while young, are parasites on other jelly-fishes. The free-swimming adult of one of them (Cunocantha octonaria) is shown in Fig. 9. It is quite



common upon the coast of Virginia and North and South Carolina. The adult is not a parasite, but as soon as the larvæ hatch from the eggs they make their way into the bell of another jelly-fish, and live there as parasites until they complete their development and assume the adult form. The jelly-fish which affords a home for these parasites is shown at k in Fig. 15. It is known as Turritopsis.