Reference for Chullin 256:43

(ויקרא יא, לב) במותם מה מיתה שאינה עושה חליפין אף כל שאינה עושה חליפין דברי רבי יוסי הגלילי

I should have said that the reason [why the flesh torn from] the living creeping thing does not render unclean was that creeping things do not convey uncleanness by carrying, but in the case of beasts, since they do convey uncleanness by carrying.

Jastrow