👉 Monocystidae is a group of marine invertebrates that belong to a family of bilaterian vertebrates. These invertebrates are characterized by their unique, single-celled structures called "monocysts," which are found on the surface or in the interior of aquatic animals such as fish, shrimp, and crabs. Monocysts can be either sessile (e.g., insects) or peristomatic (e.g., crustaceans). The term is derived from