it is surprising how even slugs come in many guises. Wiki tells us that :
Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc. The word slug is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a small internal shell, particularly sea slugs and semi slugs (this is in contrast to the common name snail, which applies to gastropods that have a coiled shell large enough that they can fully retract their soft parts into it).
Various taxonomic families of land slugs form part of several quite different evolutionary lineages, which also include snails. Thus, the various families of slugs are not closely related, despite a superficial similarity in the overall body form. The shell-less condition has arisen many times independently as an example of convergent evolution, and thus the category "slug" is polyphyletic.
its website also gives a thorough description of the parts of the slug.
LARGE BLACK SLUG : Arion ater.
These are a common sight in a variety of Scottish habitats.
The RSPB tells us that :
Slugs are similar to snails, but they have no shell. Instead they have a horny plate concealed under the mantle or saddle. Slugs are active only when the temperature is above 5 degrees C. In dry, cold weather they stay deep in the soil.
Slugs have both male and female reproductive cells (hermaphrodite), but must find a mate to exchange sperm before they can reproduce. They lay batches of gelatinous, watery eggs in moist crevices. The climatic conditions determine how quickly the eggs develop and hatch – the warmer it is, the quicker they develop. It takes about a year for slugs to mature into adults, which can live for about two years.
It should be noted that most slugs vary in colour across each species.
This one was found at the Carman hillfort in August.
JOHN INNES CENTRE slug identification guide : https://www.jic.ac.uk/research-impact/technology-research-platforms/entomology-and-insectary/slugwatch/slug-identification-guide/
WIKIPEDIA : https://en.wikipedia.org/wiki/Slug