CHAPTER IX. 



PORIFERA OR SPONGES. 



We begin the series of many-celled animals with Sponges, 

 because their average structure is simpler than that of any 

 other class. A sponge compares with sea-anemone or worm, 

 very much as a primitive community compares with a city. 

 For a sponge is a community of cells among which there is 

 little division of labour and little unified life ; in technical 

 language, it is an aggregate imperfectly integrated. With 

 the exception of the fresh-water Spongilla, the sponges live 

 in the sea, — on rocks, shells, seaweed, and the like. 



Take one of the simplest. It has the form of a cup, 

 and is moored to a rock in the sea. Its cells are arranged 

 in three strata, a thin outer skin (ectoderm), a lining of active 

 ciliated cells (endoderm) round the cavity of the cup, and 

 between these two a middle stratum (mesogloea), among the 

 cells of which lie numerous needles of lime. Put a pinch of 

 powdered carmine into the water, and you will see in part 

 how the sponge lives. For the particles are drawn in 

 through minute pores all over the surface of the sponge, they 

 pass into the cavity of the cup, and they are driven out again 

 in a stream from the large upper aperture. To what are the 

 currents due? Obviously to the lashing activity of the 

 ciliated cells. The community is Venice-like, penetrated by 

 canals. By these, food and other necessaries are continually 

 supplied to the houses or cells by the banks ; and a constant 

 current is sustained by the life of the city. 



But while this is a true sketch of more than one sponge, it 

 will not apply to all. The sponge of commerce, for instance, 

 with whose " horny " skeleton we are so familiar, is much 

 more intricate. Indeed, there are many very complex 



