Coordination in Sponges. The Foundations of Integration

Abstract

Coordination mechanisms in sponges involve not only epithelia but also the mesenchyne, which is the basic internal milieu for aii primitive Metazoa. There are three main types of coordination pathways: (i) Fluid extracellular coordination pathways are used for the spreading of materials through the mesenchymal connective matrix. Examples are provided by the processes of wound healing, regeneration, gametogenesis, and gemmulation. (ii) Mobile cellular coordination pathways follow the transitory contacts and consecutive exchanges performed by amoeboid cells which wander in the mesenchymal matrix. Examples are provided by the processes of cell reaggregation and morphogenesis, (iii) Fixed tissue coordination pathways are achieved by permanent connections between cells belonging to the same unit structure, which can be an epithelium or which can be composed of mesenchymal cell networks and bundles. Examples are provided by contractile activities of oscular membranes, internal canals, or of the whole sponge. The first and the second pathways do not undergo significant modifications in the various species of sponges. The third pathway, and especially the mesenchymal contractile tissue, appears to be significantly well developed only in thick-walled sponges. The discussion compares the three coordination systems of sponges with integrative systems in higher Metazoa. The conclusion is that if the coordination mechanisms in Porifera do not quite resemble those found in higher animals, nevertheless they would have been suitable for a further advent of true hormonal, immune, muscle, and nervous systems such as we know in other Metazoa.