Hydroid Jellyfish

Hydroids, related to the jellyfish, have three basic life-cycle stages: a tiny free-swimming planula larva, which settles and grows into a sessile attached, usually colonial hydroid stage, which in turn liberates medusae. An enlarged portion of a hydroid colony (Hydractinia) showing the nutritive polyp (a), the defensive polyp (b), and the reproductive polyp.

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