The changes of tissue appearances and structures in the process of UV-induced ＂melting＂ for sea cucumber （Stichopus japonicus） body wall were studied. And the localization and determination of acid phosphatase （ACP）, Cathepsin B and Cathepsin L activities were also investigated. The results show that the connective tissue was damaged with many hollows emerging and the regular collagen bundles were broken apart into irregular fragments. Margination of condensed chromatin at the nuclear membrane was observed. Both Golgi＇s body and endoplasmic reticulure swelled, curled, and eventually double- or multi-lamellar vesicles were formed. A number of autophagic vesicles distributed in all through the whole cytoplasm. ACP becomes more active after UV irradiation. The activities of cathepsin B and cathepsin L increased in UV-treated sea cucumbers and both achieved their maximum under certain conditions, It indicates that autophagy plays a potential role in the ＂melting＂ process for sea cucumber body wall induced by UV irradiation.
