HORNY SPONGE

any sponge of the orders Dictyoceratida and Dendroceratida (class Demospongiae). It has a skeleton consisting exclusively of fibrous organic components. Most other sponges, by contrast, have siliceous or calcareous elements as well as organic materials in their skeletal tissue. Collagen, the main organic component of horny sponges, may appear as dispersed fibrillar collagen as in other animal groups or in the form of a substance known as spongin, which is unique to the phylum Porifera. Members of the Dictyoceratida are often large, irregularly shaped sponges with a netlike skeleton of spongin fibres. The well-known commercial bath sponges (Hippospongia and Spongia) belong to this order. Horny sponges are sometimes eaten by sea slugs, starfish, and fish. Compare calcareous sponge; siliceous sponge.