Arion vulgaris is a serious agricultural pest in Europe. It is a hermaphroditic slug. They have gonad in male phase before and during mating. During the mating they “think”, that the other slug is female and they will exchange sperm reciprocally. Later, some time after mating, they will get to female phase, they will produce ova and they will use received and stored sperm to fertilize its ova.
That is not in ideal circle on the photo, because I have disturbed them to move them into glass to take photos.
Reproductive system is usually important in determination of species. There were usually black and white drawings of reproductive system of gastropods in scientific literature. Only sometimes there are photos of reproductive system in newer publications. I found no photos of reproductive system of Arion species on the internet.
End of visceral mass removed from tail end on the body of the slug. There are not parts, that could be useful in species identification in this photo. It shows hepatopancreas and ovotestis. The hepatopancreas is that brown mass. Maybe it shows the whole hepatopancreas, but I am not sure, because the whole part on the photo is roughly torned off the rest of the body. The grey part is ovotestis.
The most important part of the reproductive system in hermaphroditic pulmonate slugs is ovotestis (= gonad, = hermaphordite gland). Ovotestis consist of some lobes. Each lobe consist of number of acini. The ovotestis of Arion vulgaris (as far as I can see) has five lobes. Ovotestis produces sperm (spermatogenesis) in male phase and it produces ova (ovogenesis) in female phase. I do not know in which phase the ovotestis is on the photo. I think that it should be possible to identify histologically, but I do not know, if it is possible to identify macroscopically. Spermatozoa grows in the inner part of each acinus. Oocytes grows in cortex of each acinus.