A while ago I got curious about how much Carl Linnaeus had said about slugs in his Systema Naturae, published in 1758. Luckily, a digitized copy was readily available. Here is Linnaeus's description of the slug genus Limax from page 652 of volume 1.
Although I don't know Latin, I have been able to create a rough translation of these words with a little help from the various online Latin dictionaries and the one I pulled out of my vest pocket. What helped me most was, of course, Linnaeus's "telegraphic" style. Correct me where I am wrong.
Corpus oblongum, subtus disco plano se promovens: Body oblong, underneath in the front(?) a flat disk.
Foramen ad latus, per quod genitalia & excrementa emittuntur: Opening on the side, through which genitalia & excrements put out(?).
Tentacula supra os quatuor: Tentacles above mouth, four.
The "flat disk" underneath is the internalized vestigial shell of the slugs in the genus Limax (also present in some other genera, for example, Deroceras). The hole on the right side that Linnaeus refers to was most likely the conspicuous pneumostome, the breathing hole. The openings of the rectum and the ureter are next to it, but the genital opening is closer to the front of the head and is normally kept closed except during mating. The picture below, originally from this post, is of a couple of mating Limax maximus. It clearly shows in the slug in the back the separate locations of the genital opening from which the penis is everted and the pneumostome, the large hole to the right of it.
If Linnaeus hadn't busied himself so much with the sex lives of plants and paid some attention to the livelier and certainly slimier sexual escapades of slugs, he certainly would have corrected his little mistake.