111 PHYLUM AND CLASS PORIFERA iQl 



a portion of a living specimen of the sponge is teased out in sea- 

 water, and the broken fragments examined under a tolerably high 

 power of the microscope, groups of these collared cells will be 

 detected here and there, and in many places the movement of the 

 flagella will be readily observed. The flagellum is flexible, but 

 with a certain degree of stiffness, especially towards the base ' and 

 its movements resemble those which a very supple fishing-rod is 

 made to undergo in the act of casting a long line the movement 

 being much swifter and stronger in the one direction than in the 

 other. The direction of the stronger movement is seen, when 

 some of the cells are observed in their natural relations, to be 

 from without inwards. It is to these movements that the forma- 

 tion of the currents of water passing along the canals in due. The 

 collars of the cells in specimens teased in this way become for the 

 most part drawn back into the protoplasm. 



The short passage or excurrent canal, which leads inwards from 

 the flagellate canal to the paragastric cavity, differs from t In- 

 former in being lined by flattened cells similar to those of the 

 paragastric cavity ; it is partly separated from the flagellate canal 

 by a thin diaphragm (Fig. 72, di, and Fig. 74), perforated by a 

 large circular central aperture the apopyk(ap} which is capable 

 of being contracted or dilated : its opposite aperture of com- 

 munication with the paragastric cavity, which is very wide, is 

 termed the 'gastric ostium of the excurrent canal. 



The effect of the movement of the flagella of the cells in the 

 flagellate canals is to produce currents of water running from 

 without inwards along the canals to the paragastric cavity. This 

 causes water to be drawn inwards through the prosopyles from 

 the incurrent canals, and, indirectly, from the exterior through the 

 perforated membranes at the outer ends of the latter. 



Between the ectoderm of the outer surface and of the incurrent 

 canals and the endoderm of the inner surface and of the flagellate 

 canals are a number of spaces filled by an intermediate layer the 

 mcsodcrm or mcsoqlcca in which the spicules of the skeleton ar-- 



<J J. 



embedded. Each spicule is developed from a single cell of the 

 middle layer, the remains of the cell the sdcroUast being some- 

 times distinguishable on the surface of the fully developed spicule 

 as a thin investment. The spicules (Figs. 71 and 72, sp) are 

 regularly arranged, and connected together in such a way as to 

 protect and support the soft parts of the sponge. Most are. as 

 already noticed, of triradiate form. Large numbers, however, are 

 of simple spear-like or club-like shape ($_//); these, which are 

 termed the oxeotc spicules, project on the outer surface beyond the 

 ectoderm, and are arranged in dense- masses, one oppo-ite tin- 

 outer end of each of the ciliated canals, this arrangement pro- 

 ducing the pattern already referred to as distinguishable on tin- 

 outer surface. The thick outer layer in which the bases of these 



