Abstract:
As a large Metazoa developed, osmosis and streaming of protoplasm had to be replaced by a more efficient maintenance system. A complex arrangement of invaginations would provide sufficient absorptive area, vut no advance is possible with such a system. Circulation is an effective means of distributing materials, but a large body with an entire surface cannot absorb enough material to maintain itself. By combining systems of invaginations and circulation, however, the poor features of each system are eliminated.
