There are no germ-free animals in nature, in fact, animals live in a bacterial world. My research focuses on how interactions between metazoan hosts and their persistent symbiotic microorganisms drive body plan diversification and animal evolution (Eco-Evo-Devo). For this purpose, I use as model organisms the gutless oligochaetes Olavius algarvensis and O. ilvae. These annelids are part of a unique group of marine worms found worldwide that lack a mouth, gut, anus, and excretory organs (nephridia). They rely on bacterial symbionts for nutrition and waste disposal. By combining many “omics” with comparative developmental biology, morphology, microscopy, and in-situ hybridisation I intend to provide a better understanding of the evolution of this animal group.