Ill 



PHYLUM AND CLASS PORIFERA 



107 



the outer surface. This would appear to be due to the passage of 

 a current of water into the interior of the sponge through these 

 minute openings dotted over the surface ; and the movement of 

 the floating particles shows that a current is at the same time 

 flowing out of each of the oscula. A constant circulation of 

 water would thus be seen 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 at first sea- water, and then, when greater pressure is 



Fio. 80.— Sycon gelatinosum. Section through the wall of a cylinder taken at right angles 

 to the long axes of the canals, highly magnified ; co, collencytes ; IC, incurrent canals ; 

 ov. young ova ; Ji, radial canals ; tp. triradiate spicules. 



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 (Figs. 80 and 81, sp) of a clear 



glassy appearance. These are the calcareous spicules which form 



J 



