CIRCULATORY TISSUES, GENERAL 



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serve to remedy this condition. They have already been mentioned: 

 First, a series of one or more invaginations to increase the surface and at 

 the same time to bring it nearer to the cells to be supplied. Second, some 

 system of circulation of a fluid in the body to carry the materials more 

 easily to their destinations. 



Figure 130, C, shows a body in which the imagination of two points 

 on its surface has supplied the need, and the materials can be sufficiently 

 widely distributed. In Figure 131, D, the same principle is applied to a 



FIG. 131, D. Diagram of a still larger body with extensive system of invaginations. Sufficient 



surface but no adaptability. 



body of much greater dimensions, and it is successful as a mere distribu- 

 ter. But it can now be seen that in a body of any size the complexity 

 would become very great. Perhaps of greater importance than this is 

 the fact that each invagination would have to do exactly the same kind 

 of work that every other one did, because there would be no way for the 

 products of differentiated invaginations to be exchanged. An example 

 of an animal built on exactly such lines can be found in the sponge, and 

 such a condition constitutes in itself a form of low specialization from 

 which there is no possible advance. Evidently invagination alone is 

 not a change of structure by which much can be accomplished. 



Turning to the second of the two changes of structure that were 

 recognized as solutions of our primary difficulty, we must examine a body 



