As described throughout Section 3 (Results) and summarized in Table 2, the development of B. japonica genital tract precedes differentiation of the gonad; this is the opposite of the sequence in vertebrates such as mammals ( Gilbert, 2006 and Jost et al., 1973). This suggests that the regulatory mechanisms of endocrinological or reproductive
CGP 41251 and their functions differ between gastropods and vertebrates. In this regard, recent critical reviews of the presence of functional receptors for steroids and of enzymes for steroid synthesis or
metabolism ( Horiguchi, 2009, Scott, 2012 and Scott, 2013) have pointed out that it is doubtful whether gastropod molluscs inherently have vertebrate-type steroids as sex hormones. Further studies are needed to clarify the details of differentiation and development of endocrinological and reproductive organs such as the genital tract and gonad in B. japonica. Such studies would also help us to understand the histological changes that accompany imposex in the reproductive organs.