INTERNAL STRUCTURE. 265 



house of a large city falls down the sinks, and rolls 

 through the smaller sewers till it reaches the main, 

 and joins that of other houses, and is vomited forth 

 at the common outlet. Or like the rains and dews, 

 which, falling noiselessly and unobtrusively over a 

 great extent of country, collect in mountain springs, 

 which feed the rivulets and brooks, and these in their 

 turn unite into rivers, which open on the coast in a 

 broad estuary, and send forth a volume of fresh water, 

 whose current can be perceived for many miles in the 

 open sea. 



A Sponge is composed of a clear granular jelly, 

 investing a fibrous or spicular skeleton, formed of 

 horny matter, or flint, or lime. The Sponge which 

 we use for washing has a skeleton made up of fibres 

 of horn, but those which I have been describing have 

 their solid parts made up of flint, the particles of 

 which are arranged in needles (spicula) of a perfectly 

 transparent, solid, brittle glass. These are the rods, 

 a few of which we see projecting from the surface in 

 the Koseate and Sanguine Sponges, but which, on 

 examining a minute atom of their substance under 

 a high microscopic power, we find to be incalculably 

 numerous in the interior. The gelatinous flesh has 

 the power of secreting the flint from the sea-water, 

 and of depositing it in regular needle-like forms, and 

 in such an arrangement as to produce the canals and 

 apertures that I have described above. The flesh 

 itself is furnished, on the surface that lines the canals, 

 with curious filaments or hairs called cilia, which are 



