CHAPTER IV 



THE SPONGES 



22. Their relation to the Protozoa. While the greater 

 number of one-celled forms are not united with their fel- 

 lows, there are several species where the reverse is true. In 

 Fig. 9, for example, a fresh-water form known as Pandorina 

 is represented, consisting of sixteen cells embedded in a 

 spherical, jelly-like substance, 

 each one of which is precisely 

 like its companions in form 

 and activity. The aggregation 

 may be looked upon as a colo- 

 ny of sixteen Protozoa united 

 together to derive the benefit 

 of increased locomotion and 

 a larger amount of food in 

 consequence. As a result of 

 such a union they have not 

 lost their independence, for if 

 one be separated from the main 

 company it continues to exist. 



From such a simple colonial 



type we may pass through a series of several more complex 

 forms which reach their highest development in the beau- 

 tiful organism, Volvox (Fig. 10). In this form the indi- 

 vidual members, to the number of many thousand, are ar- 

 ranged in the shape of a hollow sphere. The united efforts 

 of the greater number, which bear on their outer surfaces 

 two flagella, drive the colony with the rolling movement 



19 



FIG. 9. Pandorina (from Nature). 

 Highly magnified. 



