The prehensile threads, now generally termed “urticating organs,” were discovered in 1835, in the Hydra, by Corda and by Ehrenberg. About the same time they were found by R. Wagner in the Actiniæ, who at first regarded them as zoosperms. Subsequently, however, he recognised their identity with the similar organs in the medusæ, and gave them all the name of urticating organs.

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