The description just given applies only to the simplest of the sponges, and we have now to learn that in the higher members of the group the structure is much more complicated. In these the surface-pores are the extremities of very narrow tubes which perforate both layers of the body-wall and then communicate with wider tubes or spaces within, some of which are lined with the ciliated cells above described. These spaces, which are sometimes nearly globular in form, and often arranged in groups with a common cavity, communicate with wider tubes which join together until, finally, they terminate in a large opening seen on the exterior of the sponge. Hence it will be seen that the water entering the minute pores of the surface has to circulate through a complicated system of channels and spaces, some of which are lined with the ciliated cells that urge the current onwards before it is expelled through the large hole. Further, imagine a number of such structures as we have described growing side by side, their masses coalescing into one whole, their inner tubes and spaces united into one complex system by numerous inter-communications, and having several large holes for the exit of the circulating water, and you then have some idea of the general nature of many of the more complex sponges to be found on our shores (see fig. 66). [Illustration: FIG. 67.--HORNY NETWORK OF A SPONGE, MAGNIFIED] But even this is not all, for as yet we have been regarding the sponges as consisting of animal matter only, whereas nearly all of them possess some kind of internal skeleton for the support of the soft, gelatinous animal substance. The skeleton consists of matter secreted by certain cells from material in the water and food, and is either horny, calcareous, or siliceous. The horny skeleton is formed of a network of fibres of a somewhat silky character, and often, as in the case of the toilet sponges, highly elastic; but it is sometimes so brittle that the sponge mass is easily broken when bent. The fibres of this framework support not only the outer wall of the sponge, but also the walls of all the internal tubes and spaces, which are often of so soft a nature that they would collapse without its aid. The other forms of skeletons consist of minute bodies of carbonate of lime or of silica, respectively, which assume certain definite shapes, resembling stars, anchors, hooks, pins, spindles, &c., and are known as _spicules_. Such spicules are usually present in those sponges that have horny skeletons, but in others they form the entire skeleton.