View through intact, translucent mantle with surface pigment scraped off.
This image shows clearly the large right nephridium (kidney) (1) because its purple-black contents contrast with the colours of the underlying viscera. On many specimens it is difficult to see when it is colourless or the same colour as the underlying viscera. It is attached to the inner surface of the mantle, so lost if the mantle is removed. It extends far onto the left of the animal. The nephridipores (openings) of it (2) and the smaller, unobtrusive, left nephridium (3) are sometimes visible by the anus (4) . Their products are conducted to the posterior of the animal by the exhalant water current.
Fertilization is external, so the male has no penis. Gametes leave both sexes through the right nephridium via its nephridiopore. The female also exudes a thin mucous film that secures the ova to the substrate.
APPENDIX re range advance/retreat: flic.kr/p/2jW74ig