CHAPTER 9 



The Phylum Porifera 



55. Introduction 



The Porifera, the phyhim of animals commonly called sponges, have 

 porous body walls and internal cavities lined with choanocytes. The 

 bulk of the body is composed of a jelly-like matrix that usually con- 

 tains a protein, calcareous or siliceous skeleton. A nervous system ap- 

 pears to be lacking. Organization among the cells is best described as 

 "loose," since cell relations can be disrupted without permanent dam- 

 age to the organism. 



Sponges are sedentary organisms ranging in size from half an inch 

 to six feet in height and varying in shape from flat, encrusting growths 

 to balls, cups, fans and vases. Most sponges are marine; only the family 

 SpongilUdae occurs in fresh water. 



The surface of the sponge is perforated with numerous small in- 

 current pores and a few large excurrent pores called oscula. These open- 

 ings are connected internally by a system of canals that includes the 

 cavities lined with choanocytes. Sponges circulate water through this 

 system and filter out microscopic food particles. In the more complex 

 sponges, which appear to have a more efficient pumping mechanism, 

 an amount of water equal to the volume of the sponge is pumped 

 through the animal each minute! 



56. General Characteristics 



The choanocytes (Fig. 9.1) are remarkably similar to the choanoflagel- 

 lates (p. 156). Each cell has a single flagellum surrounded by a delicate, 

 protoplasmic collar. As in the choanoflagellates, undulations of the 

 flagellum propel water away from the cell and occasionally bring food 

 particles against the outside of the collar. Such particles are engulfed 

 in food vacuoles and moved to the base of the cell. The layer of cho- 

 anocytes forms the sponge gastrodermis. 



In the extracellular matrix that forms the bulk of the sponge are 

 numerous, wandering, ameboid cells, the amebocytes. The amebocyte 

 is a jack-of-all-trades, secreting the gelatinous material, constructing the 

 skeleton, and gathering up debris and waste material. Some become 

 epidermal cells and form a delicate membrane over the outer surface of 

 the sponge or line the channels not already lined with choanocytes. 

 Others become muscle cells arranged around the oscula and other 

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