BDELLOURIDAE _Bdelloura._ _Syncoelidium._ TERRICOLA. BIPALIIDAE _Bipalium kewense_ Moseley (introduced). GEOPLANIDAE _Geoplana._ _Geodesmus._ RHYNCHODEMIDAE _Rhynchodemus terrestris_ O. F. M. Belonging to { _Dolichoplana._ undetermined { _Polycladus._ Families { _Microplana._ { _Leimacopsis._ TURBELLARIA. III. RHABDOCOELIDA. The Rhabdocoelida include a very heterogeneous assemblage of usually minute Turbellaria, distinguished collectively from the Polyclads and Triclads by the form of the digestive tract. This is a simple or slightly lobed sac, except in the Bothrioplanidae, which in this and many other points closely resemble the Triclads. It is to the straight, rod-like nature of the alimentary canal that the name of the group refers. The size and form of the body, and the structure of the pharynx and genitalia, vary within wide limits. The Rhabdocoelida are subdivided into three tribes:— (1) _Acoela_, in which a sub-central mouth and pharynx are present, but lead into the parenchyma of the body, not into an intestine with proper walls. An excretory system has not hitherto been seen. Yolk-glands are absent. An otolith underlies the brain. The _Acoela_ are marine. {43}(2) _Rhabdocoela_, which possess a complete alimentary tract separated from the body-wall (except for a few suspensory strands) by a space or body-cavity, filled with fluid. This space is sometimes (_Vortex viridis_) lined by an endothelium of flattened parenchymatous cells. There are two compact testes, which are enclosed (as are the ovaries and yolk-glands) in a distinct membrane. An otolith is present in some genera and species. Terrestrial, fresh-water, marine. (3) _Alloeocoela_, in which the body-cavity is greatly reduced. Except in the Bothrioplanidae, the gonads have no distinct membrane. Testes numerous; yolk-glands present. Marine with a few exceptions.