Photographic Records of Living Oscular Tubes ofLeucosolenia VariabilisI. The Choanoderm Boundary, the Choanocytes and the Pore Arrangement

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Abstract
Some ecological data concerning the Plymouth species of Leucosolenia are given, together with a discussion of the speciation of the British Leucosolenia.Oscular tubes of L. variabilis have been photomicrographed at intervals, usually of 1 or more days. It was found that the choanoderm boundary at the proximal end of the oscular rim may advance, remain stationary, or recede in relation to the distal edge of the tube, according to the conditions. It may also move in either direction in relation to the spicules in its vicinity, regardless of how it is moving with respect to the oscular edge. The evidence indicates that the choanoderm is mobile on the inner surface of the tube and that the level of the choanoderm boundary is influenced by the surface area available and by the rate of multiplication of the choanocytes. The former depends upon the degree of contraction or expansion of the tube and the size and number of pores.The arrangement of the choanocytes can alter within a few minutes. The cells, however, tend to cohere; their outlines near the base are polygonal and there is a thin hyaline layer separating the surfaces of adjacent choanocytes.The beating of the flagella is uniplanar, the plane being perpendicular to the wall. The frequency of the beat varies from one choanocyte to another, as does also the orientation of the plane of beating. The latter does not alter when a current of water is drawn across the choanoderm. Excision of an oscular tube results in water flowing out at both ends as soon as the cut has been opened.

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