It is not necessary to deal here with most of the cells that occur in the parenchyma or gelatinous part of the sponge. A full list of the kinds that are found is given by Dr. Weltner in his "Spongillidenstudien, V," p. 276 (Arch. Naturg. Berlin, lxxiii (i), 1907). One kind must, however, be briefly noticed as being of some systematic importance, namely the "bubble-cells" (fig. 2 A) that are characteristic of some species of _Ephydatia_ and other genera. These cells are comparatively large, spherical in form; each of them contains a globule of liquid which not only occupies the greater part of the cell, but forces the protoplasm to assume the form of a delicate film lining the cell-wall and covering the globule. In optical section "bubble-cells" have a certain resemblance to porocytes, but the cell is of course imperforate and not flattened. SKELETON AND SPICULES. [Illustration: Fig. 3.--Radial sections of fragments of the skeletons of _Spongillæ_. A, _S. crassissima_ var. _crassior_ (from Rajshahi); B, _S. carteri_ (from Calcutta); _a_=transverse, _b_=radiating fibres; _e_=external surface of the sponge.] In the Spongillidæ the spicules and the skeleton are more important as regards the recognition of genera and species than the soft parts. The skeleton is usually reticulate, but sometimes consists of a mass of spicules almost without arrangement. The amount of spongin present is also different in different species. The spicules in a reticulate skeleton are arranged so as to form fibres of two kinds--radiating fibres, which radiate outwards from the centre of the sponge and frequently penetrate the external membrane, and transverse fibres, which run across from one radiating fibre to another. The fibres are composed of relatively large spicules (megascleres) arranged parallel to one another, overlapping at the ends, and bound together by means of a more or less profuse secretion of spongin. In some species they are actually enclosed in a sheath of this substance. The radiating fibres are usually more distinct and stouter than the transverse ones, which are often represented by single spicules but are sometimes splayed out at the ends so as to assume in outline the form of an hour-glass (fig. 3 B). The radiating fibres frequently raise up the membrane at their free extremities just as a tent-pole does a tent. Normal spicules of the skeleton are always rod-like or needle-like, and either blunt or pointed at both ends; they are either smooth, granular, or covered with small spines. Sometimes spicules of the same type form a more or less irregular transverse network at the base or on the surface of the sponge. [Illustration: Fig. 4.--Part of an oscular collar of _Spongilla lacustris_ subsp. _reticulata_, showing arrangement of microscleres in the derma (magnified).]