PHYLUM PORIFERA 



79 



water would thus appear to be carried on — currents moved 

 by some invisible agency flowing through the walls of the 

 sponge to the central paragastric cavities, and passing out 

 again by the oscula. 



If a portion of the Sycon is firmly squeezed, there will 

 be pressed out from it first sea-water, then, when greater 

 pressure is exerted, a quantity of gelatinous-looking matter, 

 which, on being examined microscopically, proves to be 

 partly composed of a protoplasmic material consisting of 

 innumerable, usually more or less broken, cells with their 

 nuclei, and partly of a non-protoplasmic, jelly-like substance. 

 When this is all removed there remains behind a toughish, 

 felt-like material, which maintains more or less completely 

 the original shape of the sponge. This is the skeleton or 

 supporting framework. A drop of acid causes it to dissolve 

 with effervescence, showing that it consists of carbonate of 

 lime. When some of it is teased out and examined under 

 the microscope, it proves to consist of innumerable, slender, 

 mostly three-rayed microscopic bodies (Fig. 36, sp) of a 

 clear, glassy appearance. These are the calcareous spicules 

 which form the skeleton of the Sycon. 



Covering the outer surface of the sponge is a single layer 

 of flattened, scale-like cells — the ectoderm (Fig. 36, ec) — 

 •through which project regularly arranged groups of needk- 

 like and spear-like spicules {sp'). The paragastric cavities 

 are lined by a layer of cells (en) , which are like those of the 

 ectoderm in general shape ; this is the endoderm of the 

 paragastric cavity. Running radially through the thick 

 wall of the cylinders are a large number of regularly 

 arranged straight passages. Of these there are two sets, 

 those of the one set — the incurrent canals (IC) — nar- 

 rower, and lined by ectoderm similar to the ectoderm of 

 the surface ; those of the other set — the radial or 



