Board Thread:Wiki Discussion/@comment-32744161-20180311210620/@comment-47205-20180314080225

The main reason is that our classification is based on morphological features, not material ones. For that reason, the comparison between appearance (resemblance to real-world invertebrates, aquatic adaptations, presence of tentacles) is vastly more relevant than the comparison between water content.

This does mean that the current definition of Cnidaria is poor and should be modified. I shall do that immediately.

It's also worth noting that flesh is also mainly water, with muscle being over 70% water. It is true that jellyfish and well-hydrated leaves are more similar to each other in that regard, both being over 95% water, but otherwise jellyfish are obviously more similar to other animals than they are to plants.