ABSTRACT
Siphonophores (order Hydrozoans) are a direct challenge to neo-Aristotelian thesis T: all concrete biological particulars are either organisms or parts of organisms or collectives of organisms, and do not belong to more than one of these categories. Their metaphysical status has been debated for over a hundred years, with some biologists and philosophers describing them as organisms (poly-organ theory) and others as colonies (poly-person theory). The current consensus seems to be that siphonophores are a standing refutation of thesis T: they can be regarded as both colonies and organisms or even perhaps as neither, since they lie on the borderline between organisms and colonies. This chapter is a defence of thesis T in the face of the siphonophore challenge. I first outline the neo-Aristotelian position, then sketch the relevant facts about siphonophores, and go on to argue that they should be regarded as individual organisms, not colonies. I base the argument on an analysis of the structure and function of siphonophore zooids, of the question whether phylogeny is relevant to the issue of status, and of siphonophore growth and development.
