SPONGES (PORIFERA). 



Sponges differ from other animals in so many respects 

 that for along time naturalists were uncertain as to whether 

 they were animals or plants, but this matter has long been 

 settled beyond dispute. 



All sponges are composed of the following parts : On the 

 outside are numbers of small openings or pores (whence the 

 name Porifera), and these lead to small tubes or incurrent 

 canals, which extend inward to small chambers (ampullae) , 

 the digestive organs, which also contain the apparatus for 

 keeping up the flow of water through the canals. With the 

 water numerous small particles of food are drawn into the 

 ampullae, and are there taken up, while the water leaves the 

 chambers by means of a second system of tubes (excurrent 

 canals), passes into a central space (cloaca), and thence to 

 the exterior by a large opening (ostium). This same system 

 of canals also serves for respiration, but special muscular, 

 circulatory, nervous, sensory, and excretory organs are lack- 

 ing. 



In a few sponges there is no skeleton, but most species 

 have a firm support for the soft parts. This skeleton may 

 consist of small particles (spicules) of carbonate of lime or 

 of silica, often much like crystals in form ; or of fibres of a 

 horny substance; or again, both spicules and fibres may 

 occur together. In the sponges of the stores we have noth- 

 ing but the horny fibres, all of the flesh having been washed 

 away; but in this skeleton we can trace roughly the systems 

 of canals, the cloaca, and the ostium. 



Sponges reproduce by budding and by eggs. In budding 



316 



