146 PHYLUM PORIFERA 



which open to the outside. These two systems of canals com- 

 municate with each other by means of minute openings, so that 

 water which enters the incurrent canals from the outside through 

 the external incurrent pores passes freely into the radial canals, 

 and thence into the central cavity. From here it passes out 

 through the osculum. 



Exercise 2. Make a semidiagrammatic drawing of the inner surface 

 of the body wall and the cut edge of the animal, showing the fea- 

 tures described above. 



Isolate the spicules of a sponge by boiling a portion of it in a 

 caustic-potash solution. Mount some of them in water and ex- 

 amine them under a high power of the microscope. Find the 

 three different kinds of spicules : the long straight ones which 

 guard the osculum, the short straight ones which guard the ex- 

 ternal incurrent pores, and the triradiate ones which are within 

 the body wall and give it rigidity and firmness ; some of the latter 

 project into the central cavity. 



Exercise 3, Draw an outline of each kind of spicule on a large scale. 



Make thin sections of a sponge by placing it between two pieces 

 of elder pith or of cork, and shaving off the sections with a sharp 

 razor or scalpel. Obtain in this way cross, longitudinal, and tan- 

 gential sections. Mount them in water and study them under the 

 microscope. Slides of stained sections can also be used. 



Study a cross section in which the canals have been cut longi- 

 tudinally. Observe the radial and the incurrent canals and their 

 relations to one another. Note the arrangement of the spicules 

 which guard the incurrent pores, also of those triradiate spicules 

 which project into the central cavity. 



Exercise 4, Make a drawing of several canals showing these features. 



Study a tangential section in which the canals appear in cross 

 section and study the arrangement of the triradiate spicules 

 around them. 



Exercise 5. Make a drawing illustrating it. 



