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Normally, octos and cuttles will kill crustaceans by finding a joint, biting it with the beak, and injecting venom (cephalotoxin). Although most octo and cuttle venom is not too bad for humans, for crustaceans it causes their whole nervous system to go crazy, so they lose control of their muscles and are essentially paralyzed rapidly, and probably lose consciousness (although I'm not sure anyone really agrees on whether any of the various definitions of consciousness in humans apply to invertebrates-- but they certainly stop struggling and get eaten.)
In "typical" species of octo and cuttle, the beak or radula makes a hole, and the venom/saliva is actively injected from the posterior salivary glands through the salivary duct and (at least in O. vulgaris the salivary papilla. Nixon & Young describe a bit more about the system, but basically it squirts it into the hole under pressure. The venom/saliva also contains substances to soften the shell and detach the muscle/meat from the shell. I'm not sure if this system varies at all, but my impression is that it's pretty much conserved across the coleoids, I'm not sure about nautilus and Nixon & Young don't mention it in squids.