Complex sentence: The Alcyonacea, or the soft corals are an order of corals which do not produce calcium carbonate skeletons and so are neither reef-building corals nor do they lay new foundations for future corals.
Split into simpler sentences: The Alcyonacea, or the soft corals are an order of corals which do not produce calcium carbonate skeletons. Thus they are not reef-building corals and do not lay new foundations for future corals.