A pink mouthed hydroid can grow to an impressive 6 inches long. Hydroids are related to jellyfish, in the Phylum Cnidaria (pronounced ni-der-e-a). The name, cnidaria, refers to the specialized stinging cells called nematocysts. This evolutionary advancement gave rise to an entire group of animals. You can see the resemblance to jellyfish, with their clear tentacles which capture prey, floating by. Their larval stages also show a great deal of similarities.