SPONGES, AND SPONGE FISHING 59 



passes in through the inhalant canals and through the small 

 openings into the radial canals on into the cloaca and out 

 through the exhalant opening at the free end of the sponge. 

 The cells lining the radial canals and the cloaca take up and 

 digest particles of food that are brought in by the currents of 

 water. Some of this digested food is passed by osmosis to the 

 adjacent cells which do not take up any food. Here we see a 

 simple step in physiological division of labor. The outer cells, 



FIG. 13. A group of vase-shaped sponges, Lencandra apicalis. (Natural 



size.) 



which compose the ectoderm, or outer skin, serve to protect 

 the animal, while the inner cells, which make up the endoderm, 

 digest the food and feed the other members of the colony. 

 The currents that bring the food also bring fresh oxygen in the 

 water. Some of this is taken up by the cells, while the carbon 

 dioxide and other wast^products that they excrete are carried 

 away by the same currents. 



Between the ectoderm and the endoderm, and pierced by 



