22 



Sponges — A Side Line 

 or Evolution 



Cellular Organization. Sponges are living waterways. Water is constantly 

 moving over them and into and out of them, continually flowing through the 

 labyrinth of canals and chambers which they contain. These countless water- 

 courses are keys to their liveUhood. Some sponges are radially symmetrical but 

 many more have fantastically irregular forms that are named after fancied 

 resemblances, dead men's fingers, Neptune's cup, and Venus's flower basket 

 (Fig. 22.4). Water is drawn through the microscopic pores that give the name 

 Porifera to the phylum and flows through the many passageways that are 

 unique among animals. 



Protozoans are minute and unicellular while sponges are relatively large 

 and multicellular (Fig. 22.1). It is hard to find any other real difference be- 

 tween members of the two groups. The organization of sponges is relatively 

 simple, but the structure of the individual ceUs is complex and specialized. 

 Except for those that secrete the units of the skeleton, the cells carry on their 

 functions independently. Sponges have neither mouth nor digestive tract, 

 neither organs nor systems. There are no nerve cells or central controls as in 

 other multicellular animals, or as in some protozoans (Fig. 16.1). The skele- 

 ton is an outspread network of spicules or fibers. Except for the extensive 

 development of skeleton, a simple sponge resembles Proterospongia. In this 

 colonial protozoan the flagellated collared cells project from a blob of jelly in 

 which ameboid cells move about freely as they do in the jelly layer of sponges. 



Although sponge cells are relatively independent, they are also deeply 

 cooperative in maintaining the entity of the sponge and they stay together as 

 the cells of young embryos do without any apparent binding. Sponges, like 

 early human embryos, are held together by the insistent cohesion of their cells. 

 Certain sponges may be pushed through a fine cloth and their cells separated, 



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