Stem Cells: Autonomy Interactors that Emerge as Causal Agents and Legitimate Units of Selection -- Putative Stem Cell Origins in Solitary Tunicates -- Cnidarian Interstitial Cells: The Dawn of Stem Cell Research -- Stem Cells in Aquatic Invertebrates: Common Premises and Emerging Unique Themes -- Stem Cells in Asexual Reproduction of Marine Invertebrates -- Neuroimmune Chemical Messengers and Their Conservation During Evolution -- Regenerating Echinoderms: A Promise to Understand Stem Cells Potential -- Secondary Mesenchyme Cells as Potential Stem Cells of the Sea Urchin Embryo -- Cell Dynamics in Early Embryogenesis and Pluripotent Embryonic Cell Lines: From Sea Urchin to Mammals -- Regeneration in Hemichordates and Echinoderms -- Stem Cells in Sexual and Asexual Reproduction of Botryllus schlosseri (Ascidiacea, Tunicata): An Overview -- Stem Cells, Chimerism and Tolerance: Lessons from Mammals and Ascidians -- Effect of Bacterial Infection on Stem Cell Pattern in Porifera -- Defence Mechanisms and Stem Cells in Holothuria polii and Sipunculus nudus. Do real stem cells and stem cell lineages exist in lower organisms? Can stem cells from one organism parasitize the soma and/or the germ line of conspecifics? Can differentiated cells in marine organisms be re-programmed to regenerate tissues, organs and appendages through novel de-differentiation, transdifferentiation, or re-differentiation processes, leading to virtually all three germ layers, including the germline? The positive answers to above questions open a new avenue in stem cell research: the biology of stem cells in marine organisms. It is therefore unfortunate that while the literature on stem cell from terrestrial organisms is rich and expanding at an exponential rate, investigations on marine organisms' stem cells are very limited and scarce. By presenting theoretical chapters, overview essays and specific research results, this book summarises the knowledge and the hypotheses on stem cells in marine organisms through major phyla and specific model organisms. The study on stem cells from marine invertebrates may shed lights on mechanisms promoting immunity, developmental biology, regeneration and budding processes in marine invertebrates, body maintenance, aging and senescence. It aims in encouraging a larger scientific community to follow and study the novel phenomena of stem cells behaviours as depicted from the few currently studied marine invertebrates.