Post-traumatic regeneration, or the ability of organisms to grow back body parts lost to injury, is a widespread trait that has historically drawn great interest from researchers and the general public. Such interest stems primarily from the fact that in contrast with many animal groups, humans along with most other mammals are largely incapable of regrowing large or complex structures like heads or limbs. Thus, much of regeneration research is fueled by its potential to inform development of medical therapies, with aims ranging from scar-free healing to amputated limb recovery. But besides its biomedical promises, the study of regeneration also offers a window into how developmental systems can resort to different trajectories to achieve and maintain the same stable morphologies. Similarities and differences in how structures are patterned and built both during embryogenesis and regeneration can be used to better understand how developmental mechanisms and gene networks driving them have evolved. The evolution of regenerative ability is a topic of interest in itself: regeneration is a complex trait potentially subject to adaptive trade-offs, given that it brings an obvious survival advantage against accidents and sublethal predation but can also become a resource sink large enough to be selected against under certain scenarios. All these aspects of regeneration biology can be informed by comparative studies of the distribution and mechanisms of regeneration across organisms.

Regeneration is a process that can occur at a wide range of biological levels, from cells to whole organisms (Bely and Nyberg 2010). This chapter will focus on whole- structure regeneration, and thus regeneration at lower levels, such as tissues or cells, will not be covered.

Structure regeneration abilities can be classified according to the position of the structure relative to the main body axis (usually the anteroposterior axis in bilat- erian animals) in three main types: appendage regeneration, involving the loss and regrowth of structures located outside the main body axis (e.g., limbs, barbels, palps, fins, and other outgrowths); axial regeneration, which involves the loss and regrowth of structures located along the main body axis (e.g., heads, tails, and other structures removed by transverse amputations); and whole-body regeneration, in which organisms can reorganize and regrow missing structures after extensive tissue loss along any amputation plane. Axial regeneration can be further categorized according to whether regeneration can restore any length of tissue lost or is limited to restoring terminal regions only. This later distinction is particularly relevant in segmented animals that contain terminal non-segmental regions.

Within the overarching theme of this book, this chapter is organized in three parts. The first part describes the distribution of axial regenerative ability across animals. The second part presents some common patterns of regenerative processes across Metazoa. The third part focuses on current knowledge on annelid regeneration.

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