The reproductive system structure in H. obvoluta follows the helicid pattern except for the dart sac which is absent. In juvenile snails the gonad is the first to develop; the later development of male and female reproductive organs is synchronous. At the stage of lip formation the reproductive system is fully developed. At an initial stage of the gonad development (3 and 4 whorls) mitotically dividing cells prevail, and the first pools of oocytes and spermatocytes in meiotic prophase appear. At the stage of 5 whorls the number of mitoses decreases, while the number of cells in meiotic prophase increases considerably, resulting in an increase in the gonad volume. The first growing oocytes appear, while spermatids and mature spermatozoa are absent. The gonad in lip-building snails contains all the cells characteristic of mature hermaphroditic gland. In mature gonad oocytes and spermatozoa are present throughout the year. The number and size of oocytes vary seasonally. Large vitellogenic oocytes are present from April till the beginning of July and from the end of August till October. The pool of growing oocytes appears in August and in November–December. The intensity of divisions leading to formation of the respective gametes changes in a similar way. The development of female lineage cells seems to determine the timing of reproduction, since spermatozoa are present throughout the year.