Phew. The specimen illustrated is a "scalariform" variant of the common garden snail, showing a developmental abnormality where the whorls of the shell, instead of coiling right up against each other, have grown spaced apart. (First ID was by Blackbird, as well as resident malacologists Bronwen and Aydin.) In case by some incredible turn of events you don't have five thousand of them crawling around in your back garden, a normal garden snail looks like this:
The special significance of that particular scalariform specimen is that it is the type specimen of Cornu copiae, type 'species' of the genus Cornu (the image is the illustration that appeared with the original description by Ignaz von Born in 1778). If the garden snail is regarded as distinct enough from Helix pomatia, the edible snail*, to be placed in a separate genus, Cornu is the oldest genus name that has been applied to a specimen of Cornu aspersum. However, there has been a lengthy debate about whether the name Cornu is properly available, which is why some authors have used the genus names Cantareus and Cryptomphalus (the following is based on info from websites here and here).
*So called because its the species most often used for eating. Cornu aspersum is apparently also quite edible, though I've never tried myself. If you do want to eat garden snails, I'm told that you should collect them live and feed them on flour for a couple of days before eating, so that any distasteful plant matter they may have eaten themselves has time to clear from their gut.
The ICZN states that names based on "teratological specimens as such" are not treated as valid names for species (Article 1.3.2). Some authors have therefore claimed that Cornu, whose type specimen is undoubtedly teratological, is invalid. Other authors, whom I would agree with, point out the use of the words "as such" and interpret the rule to mean that names based on teratological specimens are only invalid if they were published with the knowledge that the specimen was teratological (supporting this is Article 17.3 which states that the availability of a name is not affected by its being based on "a specimen which is an unusual example of the taxon"; also, normal vs. teratological is often a matter of degree rather than clear division). The use of Latin names as labels for unusual variants of known species was common practice in the eighteenth and nineteenth centuries (and even a little bit into the twentieth; botanists still follow this practice under certain circumstances). One example of such a label that remains in use today is the reference to the black form of the peppered moth Biston betularia as the "carbonaria form". Such variant names were never intended by their original users to refer to any sort of evolutionarily or populationally distinct "species-level" taxon, which is why the ICZN excludes them. In his original description of Cornu copiae, Born gave no indication that he recognised the specimen as an unusual variant of Helix aspersa rather than a validly distinct species, so we can only assume that he genuinely believed it to be the latter (if he had specifically written something like "this unusual specimen of Helix aspersa to which I give the name Cornu copiae" it would have indicated otherwise).
Oh, and for anyone who might have been wondering why I decided to call this image ID challenge "name that bug" when the animal involved is quite evidently not a bug - i have long come to the decision that "bug" is one of the most meaningless words in the English language. Textbooks tell us that the name is supposed to only apply to insects of the clade Heteroptera (or Hemiptera, depending on the textbook), but I don't know anyone, not even among entomologists, who only ever uses the word "correctly".