58 GLIMPSES OF NATURE. 



which by its movement not only sweeps water in by 

 the pores, but sends it onwards through the sponge, 

 and in due time sends it out by the bigger holes, or 

 oscula. This constant circulation in the sponge dis- 

 charges more than one important function. For, as 

 already noted, it serves the purpose of nutrition, in 

 that the particles on which sponge-life is supported 

 are swept into the colony. 



Again, the fresh currents of water carry with them 

 the oxygen gas which is a necessity of sponge exist- 

 ence, as of human life ; while, thirdly, waste matters, 

 inevitable alike in sponge and in man as the result 

 of living, are swept out of the colony, and discharged 

 into the sea beyond. Our bit of sponge has thus 

 grown from a mere dry fragment into a living reality. 

 It is a community in which already, low as it is, the 

 work of life has come to be discharged by distinct and 

 fairly specialised beings. 



The era of new sponge-life is inaugurated by means 

 of egg-development, although there exists another 

 fashion (that of gemmules or buds) whereby out of 

 the parental substance young sponges are produced. 

 A sponge-egg develops, as do all eggs, in a definite 

 cycle. It undergoes division (fig. 1 8); its one cell 

 becomes many ; and its many cells arrange themselves 

 first of all into a cup-like form (5, 6 and 7), which 

 may remain in this shape if the sponge is a simple 

 one, or become developed into the more complex shape 

 of the sponges we know. 



In every museum you may see specimens of a 

 beautiful vase-like structure seemingly made of spun- 

 glass. This is a flinty sponge, the " Venus flower- 

 basket/' whose presence in the sponge family redeems 

 it from the charge that it contains no things of beauty 



