in PHYLUM PORIFERA 79 



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. 35, 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. 35, ec} 

 through which project regularly-arranged groups of 

 needle-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 endo- 

 derm 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 (1C) 

 narrower, and lined by ectoderm similar to the ecto- 

 derm of the surface ; those of the other set the radial 

 or flagellate canals (R) rather wider, octagonal in cross- 



