THE FRESH-WATER SPONGE. 25 



code with its supporting spicules, its pores, osteoles, and 

 communicating canals. The abundant secretion of sili- 

 ceous spicules by this simple protoplasmic mass, and the 

 arrangement of them in definite lines to form a skeletal 

 structure, are easily seen, and are somewhat remarkable 

 phenomena. 



2. The sarcode masses produced from few or many 

 separate gemmules may be seen to meet and coalesce to 

 form a single sponge, showing further the aggregate nature 

 of the animal. 



The Life Process. When the amoeba divides, its cells 

 separate as independent animals ; but when the pr.oto- 

 plasmic mass of .the sponge gemmule divides off new 

 cells, these cells remain aggregated together. Although 

 they arrange themselves so as to inclose water canals 

 when their whole mass becomes too great for contact of 

 all parts with the water, and although they cooperate in 

 the formation of spicules in more or less definite lines, 

 they yet lead in some respects an independent existence, 

 in that each cell lives, eats, assimilates, grows, excretes, 

 and divides for itself. The life process in each cell is 

 therefore not greatly different from that of the single 

 cell of amoeba, save that the sponge cell is more limited in 

 its search for food, being compelled to take such as the 

 water currents bring to it. But this loss of power to the 

 single cell is obviously to the advantage of the sponge as 

 a whole. And just in proportion as the individual cells 

 lose their individuality, and become arranged and ordered 

 for the good of the animal they compose, the animal ceases 

 to be a mere aggregate of cells, and becomes an integrate 

 organism. 



Differentiation is the term applied to the way cells have 

 of growing different, of becoming unlike in form when 



