I2 GRANTIA. 



a) A central longitudinal passage, the paragastric cavity, extending 



from the osculum to near the base, but closed at the lower end. 



b) A great many small round openings in the lining of the paragastric 



cavity. These are the gastric ostia mouths of the radial canals 

 which are outpocketings of the gastric cavity, like the fingers to a 

 glove. Their outer ends are closed and are surmounted by the 

 tufts of spicules referred to above. Similar canals show in lon- 

 gitudinal section on the cut surface of the thick walls of the 

 sponge. 



c) Running radially through the walls is also another set of passages, 



the incurrent canals. They are narrower than the radial canals 

 and alternate with them, or rather lie in the spaces between them. 

 They end blindly at their inner extremities not reaching the 

 paragastric cavity but open by three or four inhalant pores 

 at the outer end. The two sets of passages incurrent canals and 

 radial canals running side by side, communicate at certain 

 points by apertures, the prosopyles. It will not be practicable to 

 demonstrate inhalant pores or prosopyles in this exercise. By 

 means of flagella projecting from the cells lining the paragastric 

 cavity and radial canals, a current of water is drawn in through the 

 inhalant pores into the incurrent canals. From there the water 

 passes through the prosopyles into the radial canals, through the 

 gastric ostia into the paragastric cavity and out at the osculum. 

 Food and oxygen are thus supplied to the cells. Of course, it 

 will not be possible to distinguish the flagella in a dried specimen. 



Exercise 2. Draw the inner surface of a longitudinal section on a scale 

 0} five. 



Technical Note. Shave off very thin cross and tangential sections of a 

 dried sponge by placing it between two pieces of elder pith. Also 

 isolate some of the spicules by boiling portions of the sponge in a 

 solution of caustic potash. Study the tangential sections and spicules 

 with the compound microscope. Use the higher power of the dissect- 

 ing 'scope for the cross-sections. 



