CHAPTER 9 



OM 



Phylum Porifera. 

 Simple Multicellular 



Animals 



HE phylum Porifera or pore bearers are 

 commonly known as sponges. For centuries 

 they were thought to be plants, but eventu- 

 ally their animal nature was discovered. This 

 is not as strange as it seems, for some of the 

 fresh-water forms are green, due to the fact 

 that they contain many one-celled plants 

 (algae); therefore they appear distinctly 

 plantlike. Sponges are considered to be an 

 ancient group of animals that belong near 

 the bottom of the animal series but not in 

 the direct line of evolution of the more com- 

 plex animals. Even though they are not in 

 the direct line of evolution, their study is 

 of great interest for they show a multicellu- 

 lar organization that is intermediate between 

 true protozoans and typical metazoans. 



Sponges are usually attached and station- 

 ary animals in the adult stage, distribution 

 being brought about largely by the actively 

 swimming flagellated larvae, or by currents 

 of water which carr}' the young from place 

 to place before they become attached. The 

 thousands of different species vary greatly 

 in shape, size, structure, and geographic 

 distribution. Most of the poriferans, includ- 

 ing the bath sponges, live in the sea, but a 

 few belonging to a single family, Spongilli- 

 dae, are fresh-water inhabitants. 



Sponges are more complex than Protozoa, 

 and in them, division of labor among soma- 

 tic cells has resulted in myocytes and other 

 cellular specialization, but there is no group- 

 ing and coordination of specialized cells to 

 form definite tissues. Therefore, sponges 

 have not advanced beyond the cell-level of 

 organization, although they are multicellular 

 animals. Of particular interest in sponges 

 are: (1) the lack of definite germ layers so 

 characteristic of most metazoans, (2) the 

 complicated systems of canals and flagellated 

 chambers, and (3) the formation of spongin 

 5nd various types of spicules. 



92 



