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NINETEEN FIGURES INTRODPCTIOSAn understanding of the mesenchyme of the lower metazoans is essential to an interpretation of the complex sustentative tissues of the higher invertebrates and vertebrates. The rhabdocoels provide excellent material f o r such an investigation inasmuch as the animals represent the most primitive Bilateria and are one of the lower groups which have cells between the body wall and intestinal sac. Because of the phylogenetic and physiological importance of the mesenchymal derivatives of the primitive Bilateria, a recognition of their cell-types and disposition is important. The present study, then, consists of the results of an anatomical investigation of the rhabdococl Steaostoinuin virginianuwa Nuttycombe 1931, a member of the Class Turbellaria, which comprises the more primitive Bilateria.The original intention was to restrict the work to the mesenchymal elements ; however, preliminary work on the histology of the animal revealed that a detailed account of the general histology was needed. For this reason, a detailed description of the epidermis, basement membrane, muscular, digestive, and excretory systems, as well as the mesenchyme of S. uirginiamm follows.

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