Gemmules usually numerous in autumn, lying free in the sponge, spherical, variable in size but usually rather large, as a rule covered with a thick granular coat in which the spicules are arranged tangentially; a horizontal layer of spicules often present in the external membrane; the granular coat and its spicules occasionally deficient. No foraminal tubule; its place sometimes taken by an open, bowl-shaped chitinous structure the base of which is in continuity with the inner chitinous coat of the gemmule.
S. lacustris is an extremely variable species, varying in the size, proportions and shape of its spicules, in its external form and in the size and structure of the gemmule. A considerable number of varieties have been described from different parts of Europe and N. America, but some of these may represent distinct but closely-allied species; descriptions of most of them will be found in Potts's "Fresh-Water Sponges." The embryology and the earlier stages of the development from the egg have been described in great detail by Evans (Quart. J. Micr. Sci. (n. s.) xlii, p. 363 (1899)), while the anatomy and physiology are discussed by most authors who have written on these features in the Spongillidæ.
Type.—It is impossible to say who was the first authority to use the name Spongilla lacustris in the sense in which it is used by recent authors. No type can therefore be recognized.
Geographical Distribution.—S. lacustris occurs all over Europe and N. America and is probably the commonest species in most parts of both continents. It has also been found in Northern Asia and may occur in the Himalayan lakes and in the north-west of India.
1 a. Subspecies reticulata*, Annandale.
Spongilla reticulata, Annandale, Rec. Ind. Mus. i, p. 387, pl. xiv, fig. 1 (1907). Spongilla lacustris subspecies reticulata, id., P. U.S. Mus. xxxvii, p. 401 (1909).
This race differs from the typical S. lacustris in the following particulars:—
(1) The branches are always compressed and anastomose freely when well developed (fig. 5, p. [37]);
(2) the skeleton-fibres are finer;
(3) the skeleton-spicules are longer;