It IS whole.

       "What's worse than finding a Slug in your Salad?"

In the unlikely event you don't know, I'll tell you.

      "Finding HALF a Slug."

Not all that long ago I did a very rare deed, for me; I bought some Potatoes.  I tend to regard the carbohydrate part of the meal as, maybe essential, but not favourite.  Ergo I'm no great fan of Spuds, Rice, or Pasta and as for a "Chip Butty" = "A Carbohydrate Sandwich" you might as well insert a third slice of Bread.  OK, feel free to salt & vinegar it.
       The Point?  Well today I have some more of them added to a "Hentitty" (Chicken Breast) and some of the Purple Broccoli from the Garden.
      NOW can you see the point of the Slug Joke?