22 PORIFERA. 



can be easily seen that the cells that have flagella 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 (Collars may be withdrawn by cells so they pro- 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 choano- 

 cytes of the sponge resemble choanoflagellate protozoons. 



Make a drawing showing the armngement of choanocytes. 



Examine transverse sections of a specimen that has been 

 decalcified and stained. 



1. The cloacal chamber is lined by a pavement of epithelium. 



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

 the gastral 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 choan- 

 ocytes appear 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 between 

 the dermal and gastreal 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 arrows. 



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

 containing many spermatozoa, the sperin-spheres. Look also 

 for seginenting eggs, which are frequently to be found. The 

 ova are evidently formed by growth of undifferentiated 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 

 so far as the amphiblastula stage (see figures in the text-books). 

 They then break through the choanocyte layer into the radial 

 canals and pass out with the current of water. Living speci- 

 mens are frequently found with such embryos issuing from 

 the oscula in the outgoing current of water. The sperm- 

 spheres, when fully developed, also break through the cho- 

 anocyte layer and, separating into their component sperma- 

 tozoa, pass out with the outgoing water. 



