👉 The term "cystoidea" is often used to describe a type of animal that has a single kidney, typically located at the base of the pelvis. These animals are commonly found in aquatic environments such as rivers and lakes. They have a streamlined shape with a long, thin body and a large, flexible urethra for urine expulsion. The cystoidea is named after the common name "cystic" which means "fluid-filled," due to their distinctive tubular structure.