Do all creatures possess an acquired immune system of some sort?

Recent findings have provided evidence for the existence of non-vertebrate acquired immunity. We survey these findings and propose that all living organisms must express both innate and acquired immunity. This is opposed to the paradigm that only vertebrates manifest the two forms of immune mechanism; other species are thought to use innate immunity alone. We suggest new definitions of innate and acquired immunity, based on whether immune recognition molecules are encoded in the inherited genome or are generated through somatic processes. We reason that both forms of immunity are similarly ancient, and have co-evolved in response to lifestyle, cost-benefit tradeoffs and symbiosis versus parasitism. However, different species have evolved different immune solutions that are not necessarily genetically related, but serve a similar general function - allowing individuals to learn from their own immune experience; survival of species is contingent on the acquired immune experience of its individuals.