Never been a brain-eater myself, and don't think I ever will be given all of the scary prion diseases, known and unknown.
But I wanted to comment on the tentacles - back in my meat-eating days, I would often judge a sushi joint on its octupus sashimi. If it was, as you describe, slimy, tasteless and unpleasantly chewy (sort of like a bike-tire innertube with soy sauce), generally the quality of the rest of the sushi was also just so-so. But I have had some excellent octupus sashimi - light, not slimy, not overally chewy either - 'pithy' might be a better description, with a light but pleasant flavor. Of course, these were often at sushi joints of much higher quality, which also translates to sushi-joints-I-can't-afford.