CHAPTER XI 



THE SIMPLEST MANY-CELLED ANIMALS : PORIFERA 

 AND CCELENTERATA 



280. Many-Celled Animals : Metazoa. All animals higher 

 than the Protozoa consist of many cells. There is no animal 

 with simply two cells or four cells in the adult condition, 

 although all many-celled animals in developing from eggs 

 must pass through two-cell and four-cell stages, as does the 

 frog ( 59). 



All the many-celled animals taken together are termed 

 Metazoa. This is simply a convenient short term in place of 

 the phrase " animals with many cells." Familiar repre- 

 sentatives of the various types of metazoa are sponge-animals, 

 coral-animals and jelly-fishes, worms, lobster and insects, 

 clam and snail, starfish, and frog. These are some members 

 of the larger divisions (phyla) of the animal kingdom. See 

 table in 133. 



All the many-celled animals exhibit more or less physiologi- 

 cal division of labor ( 269) . In some of those which we shall 

 study first there are some groups of cells assigned to certain 

 functions, but no such separation into special organs as we 

 found in the frog. The most distinguishing feature of the 

 life-activities of the animals described in this chapter is that, 

 although each one of them may have hundreds or thousands 

 of cells in its body, they are able to live without blood or 

 similar circulating medium. We shall see that this is due 

 to their simple plan of structure. 



PORIFERA 



281. Sponge-animals. Simplest of the Metazoa are the 

 animals which produce the fibrous structures known as sponges. 



320 



