left out of the question, as the spicules are confined to a few species.

The skeleton of the horny or keratose sponges, such as the toilet sponge, is built up of horny fibres which branch and coalesce, and are interwoven into a kind of basket-work, sometimes strengthened by grains, sand, and other extraneous matter, or sometimes by a few spicules, but the most essential element is the horny fibre. In the calcareous sponges carbonate of lime, in the form of spicules combined with membrane, is the skeleton element; but in the siliceous or flinty sponges different forms and combinations of spicules cemented together form the skeleton (fig. 17).

"Sometimes the skeletons assume the shape of a beautiful regular or irregular reticulation, composed either of a nearly single series of elongate forms of spicules, cemented firmly together at their apices by keratode (which is the principal substance in the skeleton of the horny sponges), or by numerous spicules similarly cemented together, forming a strong and complicated fasciculated thread of reticulations. In other cases there is no reticulated structure, but the spicules are arranged in elongated compound bundles, which radiate from either the base or central axis of the sponge, whilst in others the reticulate and the radial system both enter into the structure of the skeleton, a modification of the network prevailing in the axis, and of the radial system towards the circumference of the sponge. Again, in other cases the spicules are simply and irregularly dispersed over the membranous base of the skeleton; and, finally, we find it simulating the form of pure keratose fibre, becoming a rigid and solid flinty fibrous skeleton."

Manifestly all this arrangement is destroyed either by the natural disintegration of the sponge, or the artificial breaking up by boiling in acid, as above stated, the cementing material being destroyed also, and nothing is left but a chaos of mixed spicules. The only method of observing the arrangement is by cutting carefully very thin sections of the sponge, and submitting them to the microscope. Of whatever substance the skeleton is composed, and however it may be combined, it is an essential of all sponge structure, as much as the bony skeleton is of the structure of a mammal or a bird.

This skeleton must be covered with flesh, not of the same kind as that of the higher animals, but of a peculiar half-transparent gelatinous substance, not altogether unlike isinglass jelly in appearance, and called sarcode. This substance corresponds to what we call flesh, but simpler in its organisation, and little more than a pellucid jelly; there is no other, or better, name which can be applied to it, since to call it flesh would be to confound it with something else, which it is not; but sarcode really means that it is "something like flesh;" therefore let it be sarcode. This substance is thinly spread all over the internal tissues, although not perfectly smooth, for sometimes it abounds in little obtuse elevations, separating occasionally into innumerable roundish or oval masses of variable size. Under a moderately high power of the microscope these masses are seen to contain granules of apparently extraneous matter in a shrivelled or collapsed condition. Occasionally there are found intermixed numerous flattened nucleated coloured cells, immersed in the sarcode.

Dr. Bowerbank says that "while the sponge, as a whole, is sensitive and amenable to disturbing causes, the sarcode does not appear to be especially so, as I have frequently observed a minute parasitical worm passing rapidly over the sarcode surface, and biting pieces out of its substance, without apparently creating the slightest sensation to the sarcode, or at all interfering with the general action of the internal organs of the sponge, and occasionally we find minute creatures permanently located in its large cavities, without appearing to cause it the slightest inconvenience."

At certain periods of the year portions of this sarcode, detached from the living animal, are capable of a certain amount of locomotion. It resembles, perhaps most nearly, the slow, gradual movements of that curious little creature, the Amœba, and consists in a constant change of form, with progress in different directions. Several independent observers have described these movements, and most of them have found them to occur at certain periods, and not at others, whatever the reason may be.

The living animal, or the units which go to make up the living animal of the sponge, has of late years been the subject of controversy, and there are still, perhaps, two principal opposed views, one in favour of an analogy with the polypes, and the other with flagellate monads. The latter view is now altogether the most general. "The true essential part of a sponge," writes one who adheres to the latter view,1 " is composed of structureless sarcode, and nucleated cells, placed side by side, with a flagellum, some cells having a hyaline collar protecting the flagellum. These latter cells line all the passages leading from

1 "On the Natural History and Histology of Sponges," by B. W. Priest, in Quekett Microscopical Journal, vol. vi. p. 232. 1881.

the pores, in most cases to the cloacal cavity, or cavities, to the oscula, regulating the currents of water, and causing them to flow through the channels, and convey the nutriment necessary to the existence of the sponge. Some naturalists, I believe, look upon the collared cells as playing the part of respiratory organs only, and not as means for assimilating nutriment; at any rate, I have no doubt, along with others, with regard to their regulating the currents of water. In some species of sponges these ciliated cells occur only in well-determined circular chambers, with their ciliated ends pointing towards the centre, each chamber having a small aperture, which perforates the investing membrane. The late Professor James Clarke, of Kentucky, was the first to notice the analogy of these ciliated cells with the free flagellate collared infusoria, followed up at the present time by Mr. Saville Kent. It has been found by this gentleman that some of the free collared monads are identical with the ciliated collared monads discovered in the sponges, each separate collar-bearing cell possessing a separate existence, and securing its nutriment in the same manner. Furthermore,1 Mr. Kent tells us that sponge structure may be, and is, built up from one of these constituent monads, by a