172 ZOOLOGY 



tect the crab by its ill odor or taste. Sponges have no power of 

 attacking any but microscopic organisms. They may, however, 

 smother by their growth fixed animals like the oyster. Because 

 of their harsh spicules or their unpleasant secretions they are 

 rarely used for food by other animals. 



The horny skeleton of certain species, the "sponges" of 

 commerce, is the chief contribution of the group to human uses. 

 Fossil sponges, apparently of the same general characteristics 

 as those now living, are found in very early geological strata. 



216. Supplementary Library Studies. 



1. Economic value of sponges. Sponge fisheries. Sponge "farming." The 

 mode of preparing sponges for market. 



2. What arguments may be advanced for considering the sponges as colonial 

 Protozoa? What is the conclusive argument for regarding them as Metazoa? 



3. By comparing the figures of sponges found in your reference books, note the 

 different degrees of development of the passages lined with entoderm and ectoderm 

 in the walls of various species. 



4. In what special ways do sponges become adapted to the conditions in which 

 they are situated? Effect of rapid currents on them? Of quiet water? Of 

 muddy water? 



