Short answers, pulled from the story.
The phylum Cnidaria contains over 11,000 species of aquatic invertebrates defined by microscopic violence from harpoon-like structures called cnidocytes. These creatures range from tiny freshwater polyps to massive lion's mane jellyfish that exceed two meters in diameter.
Fossils found in rocks formed about 580 million years ago during the Ediacaran period represent the earliest widely accepted animal fossils. Some specimens possibly date back to around 635 million years ago according to molecular clock analysis.
Jellyfish stings killed about 1,500 people during the 20th century with cubozoans being particularly dangerous. The sea wasp Chironex fleckeri is described as the world's most venomous jellyfish responsible for 67 deaths despite causing only mild symptoms in most cases.
Reef-building corals are limited to tropical seas between 30°N and 30°S with temperatures ranging from 18°C to 30°C. Fringing reefs just below low-tide level maintain a mutually beneficial relationship with mangrove forests at high-tide level.
Modern cnidarians are generally classified into four main classes including sessile Anthozoa which holds over 6,100 species and swimming Scyphozoa containing 228 species. Cubozoa represents box jellies while Hydrozoa includes freshwater forms and colonial swimmers like the Portuguese Man o' War.