In a word (of Yiddish): Traif

(pronounced "trafe" and means: not Kosher)

The rabbi I studied with boiled all the laws of Kashrut down to a four-part aphorism: "No predators; no bottom-feeders; no blood; humane killing." That's why pigs and shellfish are traif: bottom-feeders. Dogs (to stay close to topic) are, too: predators. (I don't think there were any Jews in the Corps of Discovery.)

Of course, in the days of Moses there was no Linnaean system of classification, so the generic definitions of what's permitted and what's not are a little awkward, but they've served well in spite of that. "Cleaves the hoof and chews the cud" defines perissodactylic ruminants (horsemeat and pork are both traif; venison is kosher if butchered correctly); "fins and scales" eliminates shark, shellfish and aquatic mammals in one swipe.

Accordingly, lobsters are "junk animals"... but I wouldn't go preaching that in Maine. ;-)