THE SPONGES 



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chambers and the channels which lead to and from them in the living sponges. 

 The fleshy or jelly substance of sponges is termed mesoderm (middle layer), be- 



CARPENTER'S GLASS-SPONGE, Pheronema (natural size). 



cause it is situated between the collar cells, which constitute the endoderm (inner 

 layer), and the layer of cells on the outer surface, or ectoderm (outer layer). The 

 usual flat cells of the ectoderm are now considered to be simply a superficial layer of 



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