To solve this problem, we need to identify the taxon that includes both marine and freshwater species. Let's analyze each option:$\newline \bullet$Option 1: Echinoderms$\newline$ - Echinoderms are exclusively marine animals. They are not found in freshwater environments.$\newline \bullet$Option 2: Ctenophora$\newline$ - Ctenophores, also known as comb jellies, are exclusively marine organisms. They do not inhabit freshwater environments.$\newline \bullet$Option 3: Cephalochordata$\newline$ - Cephalochordates, such as lancelets, are marine organisms. They are not found in freshwater.$\newline \bullet$Option 4: Cnidaria$\newline$ - Cnidarians include a diverse group of animals such as jellyfish, corals, and sea anemones.$\newline$ - While many cnidarians are marine, some, like certain species of hydras, are found in freshwater environments.$\newline$Therefore, the taxon that represents both marine and freshwater species is Cnidaria.$\newline$The correct option is Option 4.