Coelenterates are a group of aquatic invertebrates that include jellyfish, corals, and sea anemones. The term 'coelenterates' is derived from the Greek word for 'hollow gut, ' referring to the digestive cavity shared by all members of this group. The two crossword solutions, MEDUSAN and MEDUSAE, both refer to jellyfish, which are a type of coelenterate. MEDUSAN is a singular form of the word, while MEDUSAE is the plural form.