POKIFEEA (sponges). 



47 



together with the pores belonging to this (fig. 23, B); or it may 

 consist of a lai-ger or smaller number of such " oscula," each with its 

 proper complement of "pores" (fig. 23, A). In the latter case, 

 there is produced what is commonly called a "sponge-stock." 



In a living sponge, in its active condition, a circulation of water is 

 kept up throughout the organism by means of this canal-system. The 

 water is admitted by means of the " pores " (fig. 21, A, p ji), is driven 

 into the interior of the sponge, and is finally expelled in steady 

 streams from the osculum or oscula. The mechanism by which this 

 circulation of water is eifected was long unknown. It is now known, 

 however, that the circulation of water is maintained by the vibra- 

 tions of the flagella, with which we have seen certain of the sponge- 

 particles to be provided. In some cases, the entire system of water- 

 canals seems to be lined with these flagellate sponge - particles. 

 More commonly, the " inhalant canals " are dilated at intervals into 

 globular chambers (fig. 21, A, a a), which are lined with flagellate 

 sponge-cells. These sponge-particles are so arranged that all their 

 flagella point inwards into the centre of the chamber (fig. 21, B), and 

 in their structure they precisely resemble Flagellate Infusoria. The 

 flagella within these so-called " ciliated chambers " all work towards 

 the interior of the sponge, and thus supply the motive power needed 

 for the keeping up of the water-currents. By means of these water- 

 currents the organism obtains oxygen, and gets rid of waste matter ; 



;. 24.— A, Fragment of the skeleton of a horny sponge (after Bowerbank), showing 

 interlacing horny flhres with sxjicula. B, Different forms of the spicules of 

 sponges, much magnified. 



while the individual sponge -particles appropriate the minute par- 

 ticles of nutrient matter which may be carried in suspension in the 

 water. 



