Leviticus 11:31

These are unclean to you of all that creep,.... Unfit for food, and not to be touched, at least when dead, as in the next clause, that is, these eight sorts of creeping things before mentioned, as the Targum of Jonathan expresses it, and these only, as Maimonides says r:

whosoever doth touch them when they are dead shall be unclean until the even; for touching them while alive did not defile, only when dead; and this the Jews interpret, while they are in the case in which they died, that is, while they are moist; for, as Ben Gersom says, if they are so dry, as that they cannot return to their moisture, they do not defile; for which reason, neither the bones, nor nails, nor nerves, nor skin of these creeping things, defile; but, they say s, while the back bone is whole, and the bones cleave to it, then a creeping thing is reckoned moist, and while it is so it defiles.

r Hilchot, Abot Hatumaot, c. 4. sect. 14. s Maimon. Bartenora in Misn. Niddah, c. 7. sect. 1.