Not having a sense of dignity=/= not deserving of dignity. A demented and incontinent elderly human is certainly not dignified, but they are certainly deserving of dignity.
No, that was not my intent. Sentient means (according to Merriam-Webster): "responsive to or conscious of sense impressions" Snails are sentient, but I'm not sure how much dignity we can meaningfully bestow on them. My wife makes a living assisting "demented and incontinent elderly human"s -- so I'm not inclined to disagree on that point. I used to use the word "sapient" for what I think you're trying to capture, but that term has problems too. I've enjoyed this exchange, BTW. Thanks!
Likewise! It's been a pleasure exploring this blurry line of ethics, upon which the snail balances :)