146 



HJS'JOLOGY 



of the same size as the one which we have just been examining, in which 

 no invagination has been performed, but in which certain of the inner 

 cells have been separated to form a series of communicating channels 

 containing a fluid that can carry the materials of which we are speaking 

 and distribute or collect them. Figure 132, E, shows such a case dia- 

 grammatically. This structure has solved the single problem of distribu- 

 tion and has relieved the cells of the burden of passing materials for such 

 long distances through the body. But it has not created any more sur- 



FIG. 132, E. Diagram of a large mass of cells with perfect circulatory system but insufficient 



surface. 



face for the transference of materials, and, as the original surface was not 

 sufficient for this purpose, it can be seen that the interior cells of this 

 organism also must die. The only difference is that the inner cells will 

 all perish together instead of the innermost first. 



Let us now consider a case where a system of invaginations to increase 

 surface is developed in connection with a system of internal circulation 

 to properly distribute the materials elaborated by the surfaces of the in- 

 vaginations. Figure 133, F, shows such a condition, and it needs but a 

 minute of thought to see that the objectionable features of either in- 

 vagination alone, as in Figure 131, -D, or of circulation alone, as in Figure 

 132, E, are solved by the combination of the two. The lack of surface 

 in Figure 132, E, is provided for by the invaginations that were found 



