Thanks for your response. Only I have a specimen, and I have not any experience for dissection for small internal organs, such as nephridial bladders and ... until now.
Surely an anomalous specimen with Mp on 13! (In Lumbricidae only the Eiseniella tetraedra complex has Mp. on 13). It would be great to see the prostomium. If it is tanylobic and let's say that it has Mp. on 15, than cl: on 27-32. Typical L. rubellus. The color and the size fit but rubellus usually has a much smaller Mp. So without dissecting this is all what can be said.