
Deroceras invadens - top view, Heesch, Netherlands
Note the slime trail :)
Found on our backyard garden floor, which looks like wood, yet is actually stone. In dutch this species is named "zwervende akkerslak", which translates as Wandering Field Slug. Identification is done by an expert without much explanation, but reference photos seem to show a match. I estimate the size at about 7cm. Side view:

"Deroceras invadens" is a species of air-breathing land slug, a terrestrial pulmonate gastropod mollusc in the family Agriolimacidae.
Similar species: Land Snails And Slugs
By Ferdy Christant
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Uploaded Aug 1, 2018. Captured Jul 22, 2018 06:57.