Protozoans are at the lowest of the gradations of animality that include a wide variety of creatures.原生动物在种类繁多的动物界里是最低等的。Members of the largest and most diverse phylum of animals (Arthropoda), have segmented bodies and an external skeleton.动物界中数量和种类最多的一个门（节肢动物门）的成员躯体分节，有外骨骼。