GRANTIA. 17 



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. j\Iake out any structures you can in the area lying between 

 the dermal and gastreal layers. What cells are found here ? 



Make a draiving 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 osculum 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 present in the same individual, and the ani- 

 mal is therefore hermaphroditic. Whether self-fertilization is pre- 

 vented, as in many other hermaphroditic forms, by the ripening 

 of one element before the other, has not been fully established. 



// the time allows, draw ova, sperm-spheres, segmenting eggs, 

 and embryos. 



It is desirable to examine specimens of Lencosolenia, a still 

 simpler sponge, and of some of the more complicated forms, 

 like commercial sponges, Spongilla, Cliona, and Chalina. Why 

 is more than a single osculum desirable in such forms? Under- 

 stand the relation of the internal structure of the complicated 

 forms to the more simple forms. What reason is there for the 

 complication? 

 2 



