20 PORIFERA. 



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 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 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 sperm-spheres. Look also 

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

 ova 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 specimens 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 choanocyte layer and, separating into their com- 

 ponent spermatozoa, pass out with the outgoing water. 



Ova and sperm are formed by the same individual, and the 

 animal is therefore hermaphroditic, but the products ripen at 

 different periods and are seldom both present in an individual 

 at the same time. 



