BRANCH PORIFERA: THE SPONGES 89 



along by the waving or lashing of the flagella of the cells 

 which line the canals, and these currents of water bear 

 with them the tiny organisms which are taken up by these 

 same cells and digested. The incoming currents of water 

 meet in the central cavity or cavities of tiie body and pass 

 out through the large opening called the osculum at the 

 free end of the vase-like body, or if the body is branched, 

 through the large openings at the tips of these branches. 



The same currents of water bring also oxygen for the 

 sponge's breathing and carry away the carbonic acid gas 

 given out by the body-cells. 



As a German naturalist has said, the one necessary 

 condition for the life of a sponge is the streaming of water 

 through its body. All sponges have a system of canals 

 for this water-current and all have means, in the waving 

 flagella or cilia with which these canals are lined, for pro- 

 ducing these currents. When a live sponge is put into a 

 vessel of water, currents are immediately set up, and they 

 always flow into the body through the many fine openings 

 and out of the body through the osculum. 



Development and life- history. Although the sponge 

 in its adult condition is permanently attached by its base 

 to the sea-bottom or to some rock or shell, when it is first 

 born it is an active free-swimming creature. The sponges 

 reproduce in two ways, asexually and sexually. The 

 asexual mode of reproduction of the fresh-water sponge 

 by gemmules has already been described. The ocean 

 sponges also reproduce asexually either by forming 

 interior gemmules or external buds. In this latter 

 method a bud forms on the outer surface of the body 

 which increases in size and finally grows into a new 

 sponge individual. In some species this new sponge does 

 not become separated from the body of the mother, 

 but remains attached to it like a branch to a tree-trunk. 

 By the continued production of such non -separating indi- 



