180 



THE INDIVIDUAL ORGANISM 



Fig. 13.3. Proterospongia, a protozoan colony. Any cell in the gelatinous mass can migrate 

 to the surface and develop a flagellum and collar. (Courtesy American Museum of Natural 

 History.) 



Fig. 13.4. Diagrammatic vertical sections through simple sponge (left) and more complex 

 sponge (right). (Modified from Berry, Paleontology.) 



changing its characteristics while so doing. The inner cells are much like 

 those of Proterospongia, with food-gathering collars and whips that 

 create a water current through the sponge. These feeding cells can pass 

 on a part of their digested food to wandering cells in the interior, and 

 these in turn supply the cells of the outer layer. If the substance of a living 



