NATURE STUDY IN SCHOOLS. 



185 



As the sponge possesses the peculiar organisms explained, it will te 

 clearly seen that each of these is made up of one or more cells. For exam- 

 ple, each of the portions of the cilia are separate cells, the base being one, 

 the collar another, and the whip the third. In the flesh surrounding the 

 chambers and tubes, are also to be found many separate cells. Some of these 



Fig. 87. 



Elongated forms of sponges. 



i 



are muscle cells,; some are nerve cells, and another set are reproductive cells, 

 and as we shall ;see later, there are even male and female cells. 



It will thus 1 be seen that in the sponge there is not only a multiplicity 

 of cells, but that there is a diversity in the foim of cells. As already 

 shown the sponge began life as a single cell, and this cell was more or less 

 spherical in form. This spherical form was maintained up to the point shown 

 in fig. 86 f, when a change takes place which will be explained under re- 



