Rambam - 1 Chapter a Day

The eight crawling animals that are mentioned in the Torah are: the weasel, the mouse, the ferret, the hedgehog, the chameleon, the lizard, the snail, and the mole. The impurity of them all is the same. They are each referred to as a sheretz with regard to impurity.

A sheretz is considered as a primary source of impurity and imparts impurity to persons and keilim when touched and to an earthenware container, when it enters its inner space. It does not impart impurity when carried and one who touches it does not impart impurity to his clothes when touching it. The minimum measure that imparts impurity is a lentil-sized portion. All the sh'ratzim can be combined to reach this measure.

There is no minimum measure for the limbs of a sheretz to impart impurity. To impart impurity a limb of a sheretz must be intact as when it came into being.

A limb from a sheretz that is intact with flesh, sinews, and bones that is less than a lentil-sized portion - whether it was separated from the sheretz while it is alive or after it died - imparts impurity. It must have enough flesh or marrow within the bone to regenerate and return to a state of wholeness.

When flesh is separated from a living sheretz, it is pure. Only a limb which is comparable to an entire sheretz imparts impurity. What is meant by comparable? Just as a sheretz possesses flesh, sinews, and bones, so too, a limb separated from it must possess flesh, sinews, and bones.

A kidney, a liver, the tongue, and the like, even though they are considered as organs and will not regenerate, are considered as flesh in this context. If they were separated from a living sheretz, they are pure.

The blood of a sheretz is considered as its flesh. It can be combined to reach the minimum measure of a lentil-sized portion, as long as it is connected to its flesh.

The bones of a sheretz, its sinews, and its nails are pure. The hides of the weasel, the mouse, the ferret, and the mole are pure. This applies even though it is still fresh and was not processed or trodden upon. The hides of the hedgehog, the chameleon, the lizard, and the snail, by contrast, are considered as their flesh and a lentil-sized portion of it imparts impurity. If it was processed or trodden upon to the extent that it is processed, it is pure. For how long must one tread on a hide in order to process it? For as long as it takes to walk four millin.

When a person touches the thigh bone of a sheretz, he is pure, even though it is filled with marrow, provided the marrow in it rattles, for then it could not regenerate flesh were the sheretz to have been alive. If, however, it is in its natural place and it has sufficient marrow that it could regenerate flesh on the outside of the bone, one who touches it is impure, as is true with regard to all the limbs that could regenerate flesh as we explained. If the thigh bone was perforated to even the slightest degree, one who touches any part of it is impure.

An egg of a sheretz in which the embryo has begun to take form is pure, even though the embryonic sheretz can be seen within it. If it is perforated to the slightest degree, one who touches it is impure.

The following laws apply with regard to the carcass of a sheretz that is half flesh and half earth. If one touched the flesh, he is impure. If he touched the earth, he is pure. If the form of the sheretz was already apparent over its entire length, even one who touches the earth that has not yet been formed into a sheretz is impure.

When the flesh of a sheretz has decomposed and become spoiled and is unfit to be eaten by a dog, it is pure.

The following laws apply if the carcass of a sheretz dries out like a shard. If it would return to a moist state and be fit for a dog to eat when left to soak in lukewarm water for an entire day, it imparts impurity. If not, it is pure. It does not even impart impurity like impure foods do.

When does the above apply? With regard to a portion of a sheretz. If, however, an entire sheretz has become dry and its skeleton remains intact or it was burned and its skeleton remains intact, since its entire form remains, it imparts impurity. In my eyes, this impurity is a Rabbinic decree.

An olive-sized portion of meat from an animal carcass or a lentil-sized portion from the carcass of a sheretz that shrank and was reduced in size is pure. If a piece of flesh from a sheretz that is less than a lentil or one from an animal carcass that was less than an olive swelled and increased to the size of the minimum measure, they impart impurity according to Rabbinic decree. If, originally, they were the size of the minimum measure and then shrank, but afterwards swelled until they regained the size of the minimum measure, according to Scriptural law, they impart impurity like they did previously. Similar laws apply with regard to an olive-sized portion from a human corpse.

A sheretz does not impart impurity until it dies. If it has been decapitated, even if the head is still connected to the skin of the body, and it is still in its death throes as happens with regard to the tail of a lizard, it imparts impurity.

All the other teeming and creeping animals, e.g., a frog, a snake, a scorpion, or the like, even though it is forbidden to partake of them, do not impart impurity at all. They are not even considered as impure foods. There are no teeming animals that impart impurity except the eight species mentioned in the Torah.