Organisms Without Nervous Systems

According to current estimates, the kingdoms of life on Earth comprise at least 8.7 million eukaryotic species (protists, fungi, plants, and animals) and many more prokaryotic species (eubacteria and archaebacteria). The fuzziness of these numbers is understandable: new species are being discovered daily, and extant species are going extinct, as they always have. Whatever the number may actually be, the great majority of past and present species don’t have nervous systems. Thus, before asking what function or functions nervous systems add to animal biology, an obvious question is how organisms without them get along so well. This chapter explores the answer.