Most sponges possess a supporting framework or skeleton. In some it is formed entirely of a horny substance called spongin (as in the bath-sponge), in others it consists of spicules of inorganic matter (either calcareous or siliceous) secreted by special cells, or of such spicules bound together by spongin. Extraneous objects, such as sand-grains, are frequently included in the skeleton. The spongin is secreted like the spicules by special cells, but its chemical structure is much more complicated than that of the spicules, and it is not secreted (at any rate in most cases) in such a way as to form bodies of a definite shape. In the so-called horny sponges it resembles the chitin in which insects and other arthropods are clothed. * * * * * In no adult sponge do the collar-cells completely cover the whole of the internal surface, the olynthus being a larval form, and by no means a common larval form. It is only found in certain sponges with calcareous spicules. As the structure of the sponge becomes more complicated the collar-cells are tucked away into special pockets or chambers known as ciliated chambers, and finally the approach to these chambers, both from the external surface and from the inner or gastral cavity, takes the form of narrow tubes or canals instead of mere pores. With further complexity the simple internal cavity tends to disappear, and the sponge proliferates in such a way that more than one osculum is formed. In the class Demospongiæ, to which the sponges described in this volume belong, the whole system is extremely complicated.