UNIT AND DIVISION OF LABOR 



145 



the largest, until the side pushed in reaches the other side, 

 thus forming a cup. How many layers to the walls of the 

 cup? Note. This stage is known as the gastrula (little 

 stomach) stage, as the inner layer of cells commonly acts as 

 a feeding organ (stomach). 



Conclusions. 1. What is the main difference between a 

 single cell and a gastrula ? 



2. If the inner layer of cells of a gastrula function as a 

 stomach, where is the mouth ? Note. Practically all animals 

 above the one-celled ones pass through the stage just men- 

 tioned up to the gastrula stage, and then develop into the 

 special animal concerned. 



b. Sponges 

 1. GRANTIA 



Material. Small vials of grantia, hand lenses, toilet sponge. 



Observations. 1. Find needle-like or bristle-like structures 

 (spicules) forming the skele- 

 ton of the sponge. 



2. Can you find a system 

 of canals and cavities for the 

 passage of water ? 



3. Find a large opening at 

 the free end (the osculum). / 



4. Examine a sponge split 

 lengthwise, and one cut cross- 

 wise, and find the central 

 space or cloaca. 



5. Note the small passage 



through the body wall lead- 



& J FIG. 43. Diagram of plan of structure 



ing from the outer surface to O f a sponge. O, osculum; /, inhalent 

 the central cavity. The ex- pore ; C, cloaca, 

 ternal openings of these canals 



are called inhalent pores, and the internal openings are called 

 ostia. 



SHARPE'S LAB. MAN. 10 



