[mirimir]

Yes, they're not close relatives. Closer than us and them, but not that close. So yes, external digestion is largely a spider thing. However, crabs and lobsters do "chew" very thoroughly before swallowing. As do some spiders without external digestion.

In all invertebrates, "brains" (to the extent that they're present) are ring ganglia, with the gut going through the center. So arguably they all face that tension between brain development and the ability to eat. Vertebrates, conversely, have ~parallel guts and central nervous systems, so there's no conflict.

The thing with octopuses is that their ring ganglia are mode widely spaced. Having no exoskeleton helps with that, I guess.