Characteristics. 



CHAPTER XV. 



The Sponges, — Suhkingdom PORIFERA. 



There are about two thousand species of sponges known, which 

 range in size from a pin's head to masses several feet in height, 

 and vary in weight from a grain to over a hundred pounds. They assume an 

 endless variety of shapes, such as cups, vases, spheres, tubes, branched tree-like 

 growths, etc., but are often shapeless. When alive, they are of all colours, and 

 their consistency may be soft and glutinous, fleshy, leathery, or stony. They are 

 found in all seas, and in all depths from the shore margin to several miles deep, 

 and certain species occur in fresh waters all over the globe. About three hundred 

 species have been found round the coasts of Britain. Aristotle was the first to 

 give a scientific account of sponges. He considered that they were either animals, 

 or organisms transitional between plants and animals, and that they possessed 

 sensation, since they shrank when torn from the rocks. He classified the kinds 

 then known, and asserted that the animals often found in the cavities of sponges 

 were intruders, and did not make the sponges; further, he distinguished the 



BREAD-CRUMB SPONGE, SHOWING CURRENTS ENTERING SURFACE AND LEAVING BY OSCULES. 



large holes on the surface of certain species from the small ones, and thought that 

 water was sucked in by the former. 



From the time of Aristotle till 1762 little was recorded, but in that year 

 Ellis published his observations on the bread - crumb sponge, a British species 

 forming fleshy masses or crusts of a yellow or greenish hue. This sponge envelops 

 the stems of seaweeds, or encrusts rocks and stones ; when growing on seaweeds 

 it forms cake-like masses with a level surface, but when encrusting rocks the 

 surface is covered with small cones resembling miniature volcanic craters. The 

 surface between the craters exhibits a very fine gauze -like pattern; and, by 



