The Planarians are free-living animals, the Trematodes are parasitic upon and within animals, and the Cestodes are wholly endoparasitic.
Within this muscular tube lies a parenchymatous tissue which may be uniform (Cestodes) or differentiated into a central or digestive, and a peripheral portion (some Turbellaria), or finally the central portion becomes tubular and forms the digestive sac (Trematodes), while the peripheral portion is separated from it by a space lined in some forms by a flattened epithelium (most Planarians).
The Cestodes, however, are connected by no such intermediate forms with the other Platyelmia.
From these are given off at irregular intervals short lateral branches, each of which terminates in a flame-cell (f) precisely similar in structure to the flame-cells found in Planarians, Trematodes and Cestodes; here as there the question whether they are open to the body cavity or not must probably be answered in the negative.
Archigetes and Caryophyllaeus are the only Cestodes that become fully differentiated in an invertebrate host.