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Description of Proterospongia choanojunctaChoanoflagellate with a complex life-cycle, including a flagellated, colonial phase and a non-motile, sedentary phase composed of solitary cells reproducing themselves by means of short-lived motile swarmers. In cultures, a transition from one phase to the other demonstrable in both directions but developmental details still unknown. Colonies free swimming or attached to a substratum and varying in size from a single row or small group of cells to a domed or irregular sheet with the flagella projecting from one surface only; cell numbers ranging from 2 to several hundred. Individual cells pyriform, 3-6 microns long x 1.5-4.5 microns wide, the single emergent flagellum 12-27 microns long surrounded by a wide-angled collar 2.4-9.0 microns long, attached laterally around the periphery of the broad anterior surface subtending the flagellum. Adjacent cells in colony mutually attached by their collars (light microscopy) by means of lateral adhesion of paired tentacles (electron microscopy) but not by either protoplasmic strands or embedding material such as jelly or mucilage. Cells of sedentary phase flask-shaped, each with a close-fitting theca attached to the substratum by a small stalk 1 microns long, and with a delicately patterned anterior edge surrounding the base of the collar. The latter composed of about 20 tentacles, 5.5 microns long, attached at a wide angle near the anterior end of the cell and with other similar tentacles at the posterior end enclosed by a theca; flagellum absent, except on naked swarmers produced singly as a result of binary fission of sedentary cells. Dimensions of sedentary cells: 3.3-6.0 microns long x 2.7-4.5 microns wide. Dimensions. of swarmers: 3.0-4.8 microns x 1.8-3.3 microns, with a flagellum 12-21 microns and a short, straight-sided collar l.5-3.0 microns long and similar posterior tentacles later enclosed by a new theca when locomotion ends. Other reproductive processes not demonstrable, although some minute motile cells of unknown function sometimes seen arising from colonies, and an occasional giant sedentary cell, likewise of unknown functional significance, sometimes encountered in old cultures.