Abstract: Onchidium struma is a very important mollusk with high nutritional and pharmic value,which distribute on the mudflats of the intertidal zone of China,especially in Jiangsu,Shanghai and Zhejiang Province.Although there is an increasing demand for the aquaculture of this species,the reliable hatchery technology has not been established in China because of its complicated reproductive system and unclear early life history.Therefore,it is very urgent to understand its reproductive system and spawning process,unfortunately,only two available studies have been conducted to study the composition of its reproductive system,and no publication could be found on the histological changes of reproductive system during the spawning process of O.struma.So,the present experiments were conducted to investigate the histological changes of reproductive system between pre-and post spawning O.struma.After that,the composition and function of reproduction system(gonads and accessory sex organs) were further discussed in this paper,which should provide valuable information to understand reproductive process and improve hatchery technology of O.struma.The results indicated that:(1) The male reproductive system included penis,penial sac,male prostate,digenetic gonad(mainly produce sperm in early stage of gonad development) and sperm bursa,while female reproductive system was consisted of digenetic gonad(mainly produce mature oocytes in late stage of gonad development),gametal transportation tract,egg albumen gland,mucous gland,fertilization chamber and vagina;(2) Although there was no significant difference of male reproductive system between pre-and post spawning O.struma,the significant differences could be found on the digenetic gland,egg albumen gland and mucous gland;(3) After spawning,the digenetic gland became loose while the un-spawned mature oocyte and the early previtellogeic oocyte were found in some follicles;(4) Before spawning,the egg albumen gland consisted of many small basophilic granules which structure was very similar to the outer albumen of the embryo of O.struma.However,after spawning,the egg albumen gland contained bigger acidophilic granules than those before spawning;(5) Before spawning,there were acidophilic,basophilic and mixed area in the histological section of mucous gland while the basophilic area were dominant in the post-spawning mucous gland and the basophilic substance could be found in its secretion tube.In conclusion,these results suggested that the egg albumen gland could secrete protein to pack fertilization egg,and then to form egg albumen while the mucous gland generated mucus to encapsule and link embryo.