Note on a Sieve-like Membrane across the Oscula of a Species of Leucosolenia, with some Observations on the Histology of the Sponge
Authors: Minchin Edward A. 
Published: April 2021

ABSTRACT
At Plymouth, on the rocks immediately beneath the Marine Biological Laboratory, there are to be found at low tide a great number of calcareous and other sponges. Among them occur at least two quite distinct species of Leucosolenia, which can even be distinguished at sight by their mode of growth. The first species, which has only triradiate spicules (genus Ascetta, Haeckel), forms a network of anastomosing tubes, which at first creep close round the seaweeds and other objects, but finally, in large specimens, form great white masses of as much as two inches or more in height. From the network of tubes arise here and there the chimney-like oscula, which are simply continuations of the tubes, and not marked out by their greater diameter from them. The diameter of both the oscula and the ordinary tubes may vary within limits, but the diameter of the oscula is, if anything, less (fig. 10), at any rate not markedly greater than that of the tubes. On the other hand, in the second species of Leucosolenia, which has tri- and quadri-radiate spicules (genus Ascaltis, Haeckel), the mode of growth is essentially the same, but the oscular tubes are at once marked off from the remainder of the sponge by their very much larger size and greater diameter. They arise at intervals from the comparatively minute basal tubes, as if from a creeping stolon. Both species are of a pure white colour. The second of the two species is, without doubt, Leucosolenia botryoides (Ellis and Sol.), Bwk., the Ascaltis botryoides, var. Ellisii, of Haeckel. The first, or Ascetta species, I have more difficulty in identifying. Fig. 14, a, b, c, represents some of its spicules. As may be seen, they are precisely similar to the spicule of Leucosolenia coriacea, figured by Bowerbank.1 On the other hand, they differ somewhat from the figures of the spicules of Ascetta coriacea given by Haeckel2in having much sharper points, in which they resemble his figures of the spicules of A. primordialis.3 In his description of the spicules of A. coriacea Haeckel states that the spicules are “gar nicht, oder nnr wenig gegen die Spitze hin verdünnt. Die Spitze ist stets stumpf, niemals scharf, meistens glatt abgerundet,” &c.4 This description does not apply well to the spicules here under consideration; still less, however, does the description “schlank conisch,” applied to the spicules of Ascetta primordialis, suit them. They have much more cylindrical rays than the spicules of A. primordialis. Since, moreover, A. primordialis is said to be wanting on the Atlantic coasts, and to be replaced there by A. coriacea,5 this sponge may stand, for the present at any rate, as Leucosolenia coriacea (Montague), Bwk., the Ascetta coriacea (Tarropsis form ?) of Haeckel. It is in Leucosolenia coriacea, as here identified, that the sieve membrane occurs which I am about to describe. In the summer of 1890 I collected a quantity of this sponge in order to make preparations for teaching purposes. In vertical sections of the sponge passing through an osculum I was at once struck by the appearance of a thin perforated membrane stretched across the opening just above where the collar cells end (figs. 1, 2, 3, 5, 13). Having found no mention of such a membrane in the sponge literature1 I proceeded to investigate it further, intending at the same time to thoroughly work out the anatomy and histology of this and other Plymouth species of Leucosolenia. As, however, I was obliged to leave England for Naples at an early stage of my investigations, I thought it best to publish an account of this membrane at once, together with a few scattered observations on the histology of the sponge, hoping at some future time to make a more complete study of this interesting sponge genus.