^Chalinopsilla^ ^C. oculata^ ^C. arbuscula^ ^Euspongia^ ^E. officinalis^ Var. ^adriatica^ Var. ^mollissima^ Var. ^rotunda^ Var. ^dura^ ^Hippospongia^ ^H. equina^ Var. ^cerebriformis^ Var. ^meandriformis^ Var. ^elastica^ ^H. canaliculata^ Var. ^gossypina^ Var. ^flabellum^ Family ~SPONGIDÆ~ Subfamily ~STELOSPONGINÆ~ ^Hircinia^ ^H. campana^ [pg101] SPONGES There are many animals which consist of but one cell. These are called /Protozoa/, and comprise the /Infusoria/ and other microscopic organisms. The animals next higher in the scale are /Metazoa/, or multicellular animals, and the first group of this subdivision is =Porifera=, the sponges, the lowest of the many-celled animals. For a long time sponges occupied a disputed ground between the animal and vegetable kingdoms. Aristotle was the first to point out that a sponge is not a plant. The assertion was doubted and combated, but at last the animal nature of the sponge was established. Sponges were then believed to be colonies of one-celled animals, but finally it was decided that they were individuals with cells of different kinds that performed functions analogous to those of higher organisms. The sponge, as commonly seen, is only the skeleton or framework, so to speak, of the living animal. In its natural state it is a very different-looking object. Its entire surface is covered with a thin slimy skin, usually of a dark color, which is perforated with holes corresponding to the apertures of the canals. The organic portion of the sponge is a soft, jelly-like substance composed of three layers—the external (/ectoderm/), the internal (/endoderm/), and the middle (/mesoderm/). The external layer is composed of flat cells. The endoderm has cylindrical cells, each one of which has a flagellate hair. The main mass of the body, the mesoderm or middle layer, is made up of cells having various functions, some being concerned in the formation of framework, some in digestion, and some in reproduction.