The hydromedusan subumbrellar muscle tissues originate from a mass of endo‐ and ectoderm derived cells proliferating inwardly. This mass of cells, called entocodon, is separated by the ecto‐and endoderm through a layer of extracellular matrix, thus forming a locally triploblastic arrangement of tissues. By cavitation and differentiation, the entocodon gives rise to the striated and smooth muscle layers of the subumbrella. The structure of the striated muscle is histologically identical to that described for triploblasts, where striated muscle is mainly mesodermic. Together with the mode of development, this suggests that not all Cnidaria are diploblastic and that the subumbrellar cavity is a coelom‐like structure. The subumbreHar cavity is formed late in ontogeny, whereas the coelom in higher animals is normally formed during embryonic development. Instead of remaining closed, the subumbrella becomes open, with the muscular mesothelium in contact with the environment. This view of cnidarian structures can be pushed further while considering the regulation of modularity and metamery. The occurrence of homeotic genes in cnidarians and other ‘lower’ animals shows that the specification of body architecture has a common molecular ground throughout the animal kingdom. It is suggested that the expression of homeotic genes while specifying a given architecture might be regulated by iteration of their activity, leading to ‘low’ modular organisms and to ‘higher’ metameric ones. Replicated parts as modules and metameres might be analogous, but the affinity in their molecular specification (and possibly regulation) might indicate homology. The number of resemblances between cnidarians and ‘higher’ metazoa suggests that this phylum might be a centre of metazoan radiation, having structures which represent the living premises of the main steps of animal evolution.

We present here a morphological, cytochemical and biochemical study of the macrophagic differentiation of human pro-monocytic U937 cells exposed to moderate intensity (6 mT) static magnetic fields (MF). It was found that the following substances induced differentiation in U937 cells to a progressively lower degree: 50 ng/mL 12-0-tetradecanoyl- 13-phorbol acetate (TPA), low concentration of glutamine (0,05 mM/L), 10% dimethyl sulfoxide (DMSO) and 100 mM/L Zn++. Differentiated U937 cells shift from a round shape to a macrophage-like morphology, from suspension to adhesion growth and acquire phagocytotic activity, the cytoskeleton adapting accordingly. Exposure to static MF at 6 mT of intensity decreases the degree of differentiation for all differentiating molecules with a consequent fall in cell adhesion and increased polarization of pseudopodia and cytoplasmic protrusions. Differentiation alone, or in combination with exposure to static MFs, affects the distribution and quantity of cell surface sugar residues, the surface expression of markers of macrophage differentiation, and phagocytotic capability. Our results indicate that moderateintensity static MFs exert a considerable effect on the process of macrophage differentiation of pro-monocytic U937 cells and suggest the need for further studies to investigate the in vivo possible harmful consequences of this.