74 GENERAL BIOLOGY 



These are of two sorts, incurrent canals open to the outside, 

 and radial canals or flagellated chambers which open into 

 the central cavity. 

 Draw the sectioned specimen. 



III. Microscopic Sections. 



Study stained sections made transversely through a 

 decalcified specimen. Make a careful study of the arrange- 

 ments of the parallel tubes in the walls, and their relation 

 to the central cavity. Are the tubes open at both ends? 

 Is it possible to distinguish the incurrent and the radial 

 canals? What structural features make this distinction 

 possible? Are there any openings, the prosopyles, between 

 the incurrent and radial canals? 



With the highest power examine the cells. The central 

 cavity is lined by flat or pavement epithelium; the radial 

 canals are lined by peculiar cells, the gastral epithelium, or 

 choanocytes, which are elongated cells bearing flagella; the 

 incurrent canals and the outer surface of the body are cov- 

 ered with flattened cells, the dermal epithelium. Scattered 

 through the sections may be found germ cells, sperms or 

 eggs or segmenting eggs. Observe especially their location 

 with regard to the cellular layers. 



Make a careful drawing of a portion of a transverse 

 section. 



IV. Skeleton. 



If a specimen is boiled in caustic' potash the fleshy matter 

 will be destroyed, leaving only the skeleton. The latter is 

 made up of a series of spicules embedded in the flesh. Mount 

 some of these loose spicules in water and examine with 

 the microscope. Draw the different kinds found. 



