312 Descriptive Zoology. 



vibration of cilia propels it through the water; after it 

 becomes attached, the vibration of cilia creates water 

 currents, which bring it food and oxygen. 



Rank of Sponges. The sponges are many-celled and 

 evidently higher than the colonial protozoans. But there 

 is no high degree of differentiation of parts. The cells 

 bearing flagella and the egg and sperm cells are differ- 

 ent from the others. But there are no special distinct 

 organs for the performance of special functions. In com- 

 paring them with other forms, it may be seen that they 

 are plainly the lowest of the metazoa. 



