SYCON 39 



as thin as possible, mount in sea water under a cover, and 

 examine with a low power. This will show both incurrent 

 and radial canals in cross-section. How can you distinguish 

 one from the other? In a favorable place look for moving 

 fiagella. Are jiagella in all of the canals? In favorable situ- 

 ations it can be seen that the cells that have fiagella possess 

 collars also. (Collars may be withdrawn by cells so they pro- 

 trude but slightly.) You see now what causes the current of 

 water. Do you understand how a sponge feeds? The choan- 

 ocytes of the sponge resemble choanoflagellate protozoons. 



Make a drawing showing the arrangement of choanocytes. 



Examine transverse sections of a specimen that has been 

 decalcified and stained. 



1. The cloacal chamber is lined by a pavement of epith- 

 elium. 



2. The radial canals are lined by more conspicuous cells, 

 the gastric epithelium, or choanocytes. 



3. The incurrent canals and the outer surface of the 

 sponge are covered with flattened cells, the dermal epithelium. 



4. In a part of the section where a considerable area of 

 choanocytes appears in surface view, look for the prosopyles, 

 through which the water passes from the . incurrent to the 

 radial canals. (They may not be found.) 



5. Make out any structures you can in the area lying be- 

 tween the dermal and gastric layers. What cells are found 

 here? 



Make a drawing of several adjacent canals to show the 

 above points and indicate the course of the water by arroivs. 



6. In the stained sections, look for single ova and for 

 spheres containing many spermatozoa, the sperm spheres. 

 Look also for segmenting eggs, which are frequently to be 

 found. The ova are evidently formed by growth of undiffer- 

 entiated cells that lie between the definite cell layers and are 

 fertilized while still lying where they have developed, just 

 within the choanocyte layer. Remaining in place, they 

 undergo cleavage and develop as far as the amphiblastula 



