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The branches of Acropora corals are formed by radially symmetric skeleton lined with soft tissue. However, the transport system does not conform to the same radial structure, but is bilaterally symmetric instead. The hydroplasm flows at constant velocity towards the apex (distally) along the upper side of the branch (facing the water surface) and towards the base of the branch (proximally) along its underside (facing the seabed). The axial and the largest canal do not play a major role in the transport of hydroplasm, however, if they do conduct any movement of fluid, it is directed proximally, i.e. towards the base of the branch. The results were obtained using the simple method of vital staining of broken off fragments of Acropora branches. Changing the position of the branch for a period of several weeks did not affect the pattern of hydroplasmic flow.

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A scheme of evolution of hydrozoan colony pattern is proposed based upon the consideration of macromorphogenesis. Four main processes play decisive roles: (1) hard skeleton formation by soft tissues, (2) changes in duration of the growth phase relative to the transition to differentiation in interdependent zones of growth, (3) ratio in growth rates between adjacent zones of growth within the rudiment, the shoot, or the whole colony, and (4) spatial relationships among growth zones. The main tendency in morphological evolution of the hydroids is an increasing integration of the colony as revealed by increasing complexity of its structure. That is from a temporary colony towards the permanent one with highly organised shoots, as hydranths and branches are localised in a strictly arranged manner. An analysis of diverse data allows one to state that the main morphogenetic mechanism of increasing complexity in the hydroid colony is convergence, then fusion, of adjacent growth zones, a variant of heterochrony.