SPONGES 
HERE are many animals which consist of but one cell. These 
are called Protozoa, and comprise the Infusoria and other mi- 
croscopic organisms. The animals next higher in the scale are 
Metazoa, or multicellular animals, and the first group of this sub- 
division 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 indi- 
viduals with cells of different kinds that performed functions 
analogous to those of higher organisms. 
The sponge, as commonly seen, is only the skeleton or frame- 
work, so to speak, of the living animal. In its natural state it is 
avery 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 diges- 
tion, and some in reproduction. 
The framework is secreted in the mesoderm, and in different 
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