The triclad, Polycelis nigra, has been found to be fully cellular. Gland-cells, undifferentiated cells, and the cell-bodies of muscle-cells, make up the parenchyma. The fine structure of the component cells of the parenchyma, nervous, and excretory systems, testis, pharynx, and epidermis is described. Acidophil secretion granules, produced by certain parenchymatous gland-cells, have a characteristic, doubly-banded ultrastructure which is not invariably associated with the property of adhesiveness. The parenchymatous cell-body of the muscles is often up to 10 µ. from the musclefibre, to which it is joined by tenuous cytoplasmic connexions. The muscle-fibre itself consists of coarse and fine sets of hexagonally arranged myofilaments, but is unhanded. The basement membrane of the epidermis is composed of fine, banded fibrils, apparently randomly arranged in the plane of the membrane. Permeating the epidermis at a level just above the basement membrane is a system of extracellular spaces, which may have a hydrostatic function and assist in the extrusion of secretion granules. Epidermal sense organs, whose fine structure resembles the basal body of the cilia, are considered to have a functionally significant distribution on the surface of the animal. The rhabdites have been shown to develop in special cells of the parenchyma. Such rhabdite-forming cells, together with their contained rhabdites, have been found apparently passing through the basement membrane of the epidermis. As all the epidermal epithelial cells contain rhabdites, it is suggested that the epidermis as a whole is renewed by centrifugal migration of rhabdite-forming cells. The rhabdites themselves appear to consist of arginine and some tyrosine, together with a purine, probably adenine. They may be an excretory product.

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