CHAPTER 8 



THE SPONGES AND THE TWO-LAYERED 



ANIMALS 



PHYLUM PORIFERA 



The sponges constitute the phyhnn 

 Porifera, which means "pore bearer," the 

 presence of pores being one of the charac- 

 teristics of the group. This rather unique 

 group of very simple animals is not in the 

 direct line of ascent to higher forms and is 

 sometimes placed in a separate subking- 

 dom, the Parazoa. Sponges possess flagel- 

 lated collared cells which resemble some of 

 the protozoan forms (Fig. 7-16), indicating 

 that they may not be far removed from the 

 one-celled group. Digestion is wholly intra- 



cellular and, except for simple epithelia, 

 there is no arrangement of cells that can be 

 considered tissue. For this reason sponges 

 are usually considered to be of the cellular 

 grade of organization. 



Living sponges have a gelatinous tex- 

 ture, quite different from the familiar bath 

 sponge sold on the market. Being sessile, 

 they are easily mistaken for plants. Sponges 

 assume many shapes, vary in height from 1 

 millimeter to more than a meter, and are 

 usually drab in color. Some forms, however, 

 take on shades of red, yellow, blue, black, 

 or green, the last being caused by the green 



136 



