The level of consciousness in something like a brain parasite or a slug is probably so dim that it barely feels like anything to be one. So even if you were reincarnated as one, you likely wouldn’t have much of a subjective experience of it. The only way to really experience a new life after reincarnation would be to come back as something with a complex enough mind to actually have a vivid sense of existence. Not that it matters much - it’s not like you’d remember any of your past lives anyway.