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Description of Monosiga ovataThe ovoid cell is approximately 4 microns long or 15 microns when measured from the tip of the flagellum to the distal end of the stalk. At the anterior end of the cell a ring of 20-25 tentacles, almost equal in length, forms a collar which encircles the single flagellum. The latter projects beyond the tentacles and terminates in a conspicuous hair point. The cell body and the base of the tentacles are closely invested by a delicate membranous sheath and this tapers posteriorly to form the stalk or peduncle which attaches the cell to the substratum. The apparent looseness of the sheath in shadowcast whole mounts is probably caused by the shrinkage of the protoplast during drying. Transverse sections of the cell in the region of the tentacles and cell body show that the sheath fits closely although it is ridged at intervals. That part forming the stalk sometimes has a fibrillar appearance but it is not clear whether this is a partial disintegration caused by drying or a genuine arrangement for attaching the cell to the substratum. In section the sheath is seen to be composed of two regularly spaced layers, the surfaces of which are covered with a fine fibrillar deposit possibly of mucilage.