SPONGES 



rTlHERE are many animals which consist of but one cell. These 

 A. 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 

 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 diges- 

 tion, and some in reproduction. 



The framework is secreted in the mesoderm, and in different 



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