Black/Spanish/Large Red Slug Arion ater/vulgaris/rufus agg.
These three species are very difficult to tell apart in the field, added to which there is some evidence they may be able to hybridise. Hence, all on one page.
Earlham Cemetery, Norwich - May 2021
This is a very confusing species that can only really be differentiated by dissection. As can be seen from this picture, they're not all black either. Therefore, it's common to record them as an aggregate (agg.) species complex. Norwich - September 2019
Deerness, Orkney - August 2017