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 How many mature hydranths are seen in the hydroid shown in Fig. 40? Why are the defensive hydranths on the outside of the colony? Which hydranths have no tentacles? Why not?

Jellyfish.—Alternation of Generations.—Medusa.—With some species of hydroids, a very curious thing happens.—The hydranth that is to produce the eggs falls off and becomes independent of the colony. More surprising still, its appearance changes entirely and instead of being hydra-like, it becomes the large and complex creature called jellyfish (Fig. 43). But the egg of the jellyfish produces a small hydra-like animal which gives rise by budding to a hydroid, and the cycle is complete.

(medusa).

The bud (or reproductive hydranth) of the hydroid