I think the distinction being made is between evolution and design. In evolution traits do not come about for a reason, they occur at random, but they persist if they are advantageous.
Yes. But there's an illusion of intentionality. Because [pausing, looking at my hands] it's all just so bloody amazing.
But on the other hand, some of the kludginess is also bloody amazing. Spiders, for example. The gut passes through the "brain". So there's a tradeoff between "brain" size and eating. And so they employ external digestion, and suck liquid.
Well, a couple of things start to make sense about the kludginess, if you start with the single-cellular origin, and then pile up layers of specialized tissue, with some cells permanently buried within the organism, never to be exposed to an external environment.
The first part is that brains are just plain weird, and perhaps one of the weirdest aspected of any multicellular organism. More than half of all known life forms don’t have brains. So, brains usually aren’t a priority.
Second, the gut is often regarded as a brain-like organ in many vertebrates, and so, with arthropods and crustaceans, it’s interesting that the the two might be more tightly coupled. It probably cheapens the endocrine feedback loop.
Third, external digestion actually comes across as less freakish and uncivilized underwater. On land it has a messy, disgusting sensibility, but in the ocean it’s kind of on par with a smokey cooking fire at a camp site, especially when considered from a single celled animal’s perspective.
When you consider that crabs and lobsters are the ancestral relatives of insects and arachnids, the low prioritization of a brain, and externalized digestion start to look more and more normal.
Crabs and lobsters don't use external digestion but eat their food not terribly different from what we do. So it looks like spiders evolved their system later.
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.
But on the other hand, some of the kludginess is also bloody amazing. Spiders, for example. The gut passes through the "brain". So there's a tradeoff between "brain" size and eating. And so they employ external digestion, and suck liquid.