ABSTRACT
During arm autotomy the aboral integument of Asterias rubens constricts and ruptures at a localised plane. This paper describes the histology and ultrastructure of the autotomy plane, and the mechanical behaviour and responses to chemical agents of isolated preparations that include the plane. The autotomy plane is characterised by the presence of a band of circular subepidermal muscle fibres and by a structurally distinct region in the upper dermis which may act as a preformed weak-point. It is shown that the lower dermis undergoes an endogenous loss of tensile strength at autotomy and suggested that this might involve granule-containing cell processes of which three types can be distinguished. In creep tests, isolated preparations of integument including the autotomy plane show a lower viscosity than preparations of more distal integument. Excess K+ ions reduce and acetylcholine increases their viscosity. Pharmacological evidence indicates that the latter response may involve a muscarinic pathway. The results provide further evidence for the central role of mutable collagenous tissues in echinoderm autotomy mechanisms.
