Ectoproct colonies with membraniporiform, celleporiform and Iunulitiform A growth‐forms have particular habitat preferences on the continental shelf. A colonial capability model for each growth‐form, based on a functional analysis of its structure at different levels, tries to portray the adaptive range of the growth‐form and includes inferences from the structure of the individual zooids, their polymorphs, their arrangement within the colony as controlled by budding patterns, and their relation to external environmental stresses. Each encrusting growth‐form on the Sahul Shelf, northwest Australia, has distinct structural and functional characters which can be arranged in adaptive architectural trends. Deviations from the fundamental membraniporiform growth‐form include the development of vertical growth, the sacrifice of individual zooids, the incorporation of substrate within the colony, and variations in the heterozooids. The celleporiform growth‐form is subdivided to recognize the massive, ‘nodular’ colony as a s