182 INVERTEBRATE ZOOLOGY 



canals, which are cut longitudinally here. Notice also the 

 shorter and less regular incurrent canals, which lie between the 

 radial canals and open to the outside through external incurrent 

 pores. There are thus two systems of canals in the body-wall, 



(a) the radial canals, which are a part of the central cavity, and 



(b) the incurrent canals, which open to the outside. These two 

 systems of canals communicate 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 features above described. 



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

 caustic potash solution. Mount some of them in water and 

 examine 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 

 external 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. Determine whether the 

 spicules are solid or hollow. 



Exercise 3. Draw an outline of each sort 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 tangential sections. Mount them in dilute glycerine and 

 study them under the microscope. 



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

 gitudinally. Observe the radial and the incurrent canals and 



