82 MANUAL OF ZOOLOGY SECT. 



will be readily observed. It is to these movements that the 

 formation of the currents of water passing along the canals 

 is due. 



The short passage or excurrent canal, which leads inwards 

 from the flagellate canal to the paragastric cavity, differs 

 from the 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. 32, di.\ 

 perforated by a large circular central aperture the apopyle 

 (ap.) which is capable of being contracted or dilated; its 

 opposite aperture of communication 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 mesoderm or mesoglaa in which 

 the spicules of the skeleton are embedded. The 

 spicules (Fig. 35, sp.\ each of which is developed in a 

 single cell of the middle layer, 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 (sp'.). The sexual reproductive 

 cells the ova (Fig. 32, ov.} and sperms are developed 

 immediately below the flagellate endoderm cells of the 



