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 
(6) 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. 
Bxercise 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 ina 
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 
