The relationship between body weight and gonad development of Onchidium struma
was studied using a histology method. The relation between the gonadsomatic index (GSI) and hepatopancreas somatic index (HSI) or vitelline somatic index (VSI) were also studied. The composition of germ cells and the percentage difference of phases of germ cells in gonads were analyzed. Spermatogensis and oogenesis of O. struma
were observed. The results showed: (1) The development of gonads in O. struma
matured as body weight increased. The GSI and HIS or VSI had obvious pertinence respectively, (P<0.05）. (2) The oocytes appeared in individuals whose body weight was above 6 g and from this time the oocytes and sperm cells coexisted in the gonads. (3) The spermatozoa were found in all individuals. The main germ cells were the secondary spermatocyte in individuals whose body weight fell under 6 g and those with body weights above 6 g had spermatozoa. The individuals who were above 6 g; the oocytes matured as body weight increased. The main elements of the germ cells were oogoia (57%), exogeous vitellogenic oocyte(69%) in the 6-8g individuals and 10-14 g individuals respectively. The mature oocytes were first found in the 10-14 g individuals. (4) The oogenesis can be divided into 6 stages: oogoia，previtellogeic oocyte，endogenous vitellogeic oocyte，exogeous vitellogenic oocyte，nearly-mature oocyte and mature oocyte (Φ=59.358±3.88 μm). The spermatogensis consisted of five stages: spermatogonia，primary spermatocyte，secondary spermatocyte and spermatozoa (52.44±20.65 μm). We can conclude our research in two points: (1)There is a relationship between body weight and development of gonads. (2) The individual with a body weight above 10 g can be a parent.