Autotomy as a prelude to regeneration in echinoderms.

'Autotomy' refers to the adaptive detachment of animal body parts where this serves a defensive function, is achieved by an intrinsic mechanism, and is nervously mediated. With regard to each echinoderm class, this article itemises those structures that are autotomous, evaluates the extent to which autotomy precedes regeneration in natural populations, reviews current knowledge of the morphology of autotomy planes and mechanisms that effect fracture at autotomy, and comments on autotomy-related issues arising from studies of the cellular events of regeneration. Each autotomy plane can be regarded as an assemblage of breakage zones traversing the individual anatomical components of the autotomous structure. In any one autotomy plane some breakage zones are permanent sites of weakness that are fractured by external forces and some are potential sites of weakness that undergo a loss of tensile strength only at the time of autotomy. The latter occur predominantly in mutable collagenous structures, although there are a few examples of muscles that undergo an endogenous rupturing process. Available evidence indicates that autotomy is by far the commonest proximate cause of structural loss in echinoderms. Most echinoderm regeneration is therefore necessitated by autotomy and proceeds from the retained side of a fractured autotomy plane. Due to a lack of relevant research there is as yet little evidence for or against the presence of specific regeneration-promoting adaptations at autotomy planes, although it is argued that an autotomy plane designed primarily to effect rapid detachment would by itself increase regenerative efficiency.