TObat 10 a Sponge? 173 



in little sacks until they have reached a cer- 

 tain stage of progress, when they are expelled 

 from the mother sponge and turned adrift in 

 the great ocean to struggle for their own exist- 

 ence. These eggs do not differ much in their 

 structure and composition from an ordinary 

 hen's egg, except that there is no shell, only a 

 skin provided with little fibers called cilia, 

 that project from it, and by the movement of 

 these the embryo sponge is able to propel itself 

 through the water. It thus lives until it has 

 reached a certain stage of development, when 

 it seeks out a pebble or rock, to which it at- 

 taches itself at one end preparation for 

 which has been made by its peculiar structure 

 during its life when it was free to float around 

 through the water. It is now a prisoner and 

 chained to the rock it has selected for the 

 foundation of its home. Having no longer 

 any use for the little cilia, which enabled it 

 to swim through the water, it now loses them. 

 Here is a beautiful illustration of how nature 

 provides for the necessities of the smallest 

 things, and how when the necessity that de- 

 manded a certain condition passes by the con- 

 dition passes with it. The embryo begins to 

 show a fibrous development, which is the be- 

 ginning of the framework of a new sponge. 

 Evolution goes on, every step of which is as 

 mysterious as a miracle, until the growing 

 thing is a full-grown sponge, equipped with 



