SPONGE, the horny skeleton, formed of keratose, or i spongin, produced by certain ani mals living n the sea and of very low grade in the scale of life. Down to a quite recent period they were classified as colonial Protozoa, the lowest group of animals; but all naturalists now them as separate phylum under the name Portfera (or Spongiaria), given to them because of the water-pores with which they are provided in such abundance. These pores are ordinarily and chiefly of two kinds, both open ing into the internal or digestive canals and passages which everywhere ramify through the mass of the sponge. There are large openings (oscula) relatively few in number, often guarded by special protective devices such as circles of spicules or muscles capable of con tracting the orifice, and much more numerous small pores everywhere perforating the surface. The latter are inhalent pores or inlets, which admit water to the internal passages, from which it escapes through the former. A con stant current is induced by the activity of count less flagella or little living lashes, generally confined to definite enlargements of the canals known as ciliated chambers which constantly beat the water and drive it ever onward in one direction. This current is a veritable life giving one to the sponge, bearing into the diges tive cavities not only the minute organisms upon which the sponge animal feeds, but also supply ing oxygen for respiration, bearing away all waste and excreted substances and assisting in the fertilization of the eggs and the distribution of the larva. Some of the simplest sponges, such as are found among those with calcareous skeletons, have but a single osculum at the summit of a sac-shaped or cylindrical body, the walls of which are perforated by many radiating canals, each opening to the exterior by an in halent pore and ending internally in the central digestive chamber. The walls of such a sponge are composed of the sponge-flesh, formerly called sarcode, together with the skeleton secreted by certain cells of the latter. 'The flesh consists of three chiefly cellular layers, ectoderm, mesoderm and endoderm. The ectoderm is the sponge
slain and, in the form of a layer of flat cells, covers the whole exterior and lines certain spaces or chambers which are formed in many sponges by a process of enfolding. The en doderm lines all of the truly internal chambers and passages, and consists partly of flattened digestive cells which line the central cavity and parts of the radial canals, and partly of cells which bear each a flagellum with a collar sur rounding its base on their free ends, and which are confined to the ciliated chambers. These lat ter collared and flagellated cells are very char acteristic of sponges and are found elsewhere only among the Protozoa. Between these limit ing layers the mesoderm makes up the great bulk of the sponge-body and consists of a variety of different kinds of cells, of which the most important are the reproductive, from which the eggs and spermatozoa are produced; and the skeletogenous, which secrete the ele ments of the skeleton.
The great majority of sponges are originally of much the form just described, which may be considered as exemplifying the sponge in dividual; but as they grow they bud and branch in a very plant-like manner, forming many new oscula and many new partial individuals which often reunite and enclose cavities lined with ectoderm. In this manner colonies of large size and most varied forms are built up, and, simple as the fundamental sponge plan is, there are few animals which present more complex struc tures and whose morphology has been so late in being correctly explained. The skeletons of sponges may be calcareous or silicious or horny, or the latter may be combined with either of the others. The first two may exist in the form of separate spicules, presenting the most varied shapes characterizing the different genera and species, or built up into a more or less continu ous frame-work. Horny or sponging skeletons consist of fibres, almost always interlacing, branching and uniting in a most complete and complex manner, and sometimes strengthened by included silicious of calcareous spicules or granules.