108 



ZOOLOGY 



SEOT. 



fi 



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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 

 exerted, a quantity of gelatinous-looking matter, which, on being 



examined micro- 

 scopically,proves 

 to be partly com- 

 posed of a proto- 

 plasmic material 

 consisting of in- 

 numerable usu- 

 ally more or less 

 broken cells with 

 their nuclei, and 

 partly of a non- 

 proto p 1 a s m i c, 

 jelly - like sub- 

 stance. When 

 this is all re- 

 moved there re- 

 mains behind a 

 toughish felt-like 

 material, which 

 maintains more 

 or less complete- 

 ly the original 



n 



FIG. 81. Sycon gelatinosum. Section throughthe wall of a 

 cylinder taken at right angles to the long axes of the canals, 

 highly magnified ; co, collencytes ; 1C, incurrent canals ; or. 

 young ova ; R, radial canals ; sp. triradiate spicules. 



shape of the sponge. This is the skeleton or supporting frame- 

 work. 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. 81 and 82, sp) of a clear glassy appearance. These 

 are the calcareous spicules which form the skeleton of the Sycon. 



The arrangement of the spicules, their relation to the proto- 

 plasmic parts, and the structure of the latter, have to be studied 

 in thin sections of hardened specimens (Figs. 81 and 82). An 

 examination of such sections leads to the following results. 



Microscopic Structure. Covering the outer surface of 

 the sponge is a single layer of cells the dermal layer or 



