The œcological significance of cilia and ciliary currents has been studied hitherto mainly so far as they are connected either with the epidermis or with the intestine. Epidermal cilia produce currents by which either the animal itself is moved or the surrounding medium is carried over the surface of the unmoved animal. In the latter case the ciliary currents serve for collecting substances, ordinary food and oxygen. It may be said, that the degree of specialisation of epidermal cilia generally indicates the degree of intimate dependence of the animal on its outside conditions (natural surroundings). (The cases where epidermal cilia are connected with sense organs are comparatively few.)