A description of the fresh-water sponge already given may serve very well to indicate the general outlines of the organisation of the marine ones too. The spicules of the latter are, however, not always flinty; very often, as in the case of _Grantia_ (Plate IX. Figs. 8 and 14), they are calcareous, a point which can be settled by the application of a little nitric acid and water. If lime be present there will be strong effervescence, and the separation of the spicules can only be effected by gently warming a portion of the sponge in caustic potash solution, pouring the resulting mass into water, and allowing the spicules to settle. The washing and settling must be repeated several times, and a portion of the deposit may then be taken up with a dipping-tube, spread upon a slide and dried, and then covered in balsam solution. The forms are endless, and the same sponge will often supply three or four, or even more. Among them may be seen accurate likenesses of pins, needles, marlin-spikes, cucumbers, grappling-hooks, fish-hooks, porters’-hooks, calthrops, knife-rests, fish-spears, barbed arrows, spiked globes, war-clubs, boomerangs, life-preservers, and many other indescribable forms. The flinty forms must be prepared by boiling, as described in speaking of the mounting of diatoms in Chapter XI., except that, of course, only one settlement is required after thorough washing. Every one who has been by or on the sea on a fine summer night must have noticed the bright flashes of light that appear whenever its surface is disturbed; the wake of a boat, for example, leaving a luminous track as far as the eye can reach. This phosphorescence is caused by many animals resident in the sea, but chiefly by the little creature represented at Fig. 9, the _Noctilúca_, myriads of which may be found in a pail of water dipped at random from the glowing waves. A tooth of this creature more magnified is shown immediately above. A large group of microscopic organisms is known to zoologists under the name of Foraminifera, on account of the numerous holes in their beautiful shells, most of which are composed of carbonate of lime, though some are horny and others are composed of aggregations of minute grains of sand, the forms in one class often closely imitating those in another. It is of the shells of these minute animals that the “white cliffs of old England” are very largely composed, and those who desire to understand the part which these tiny creatures have played, and are playing, in geology, will do well to study Huxley’s fascinating essay on “A Piece of Chalk.” XIII.