228 THE AMERICAN MUSEUM JOURNAL 



greatly in thickness, and the canal system may become much 

 complicated through the folding of the walls of the paragastric 

 cavity and the development of wide mesodermal cavities {JAih. 

 and exh^. 



3. The Skeleton. 



One of the most remarkable features of sponge structure is 

 the skeleton. It is by far the most reliable basis for classifying 

 the sponges yet discovered, inasmuch as it is comparatively 

 unaffected by the external surroundings of the individual and 



FIG. 12. — A BAHAMAN COMMERCIAL SPONGE (//^>/''-f/<'«^'<J J/. Hyatt) 

 Showing colony of tube-like individuals 



therefore its peculiar features remain constant to the groups of 

 which they are characteristic. It may be composed either of 

 fibers or spicules, and it is secreted by the mesoderm. Its 

 function is to furnish a rigid supporting framework for the 

 body and to act as a protection against the enemies of the sponge. 

 The fibrous sponges include among others those known to 

 commerce. The skeleton is, in most cases, made up of interlacing 

 and anastomosing fibers of a homy substance called spongin, 



