Mammals, birds, fish, and even some locusts are kosher only if they meet the exacting criteria set out in the Bible and rabbinic law.
There are similar problems regarding turbot and swordfish. Conservative Rabbis have ruled that swordfish is kosher, since the Talmud states explicitly that it is kosher. Most Orthodox Rabbis, however, are doubtful whether the fish mentioned in the Talmud as kosher is the swordfish. English rabbis in the nineteenth century ruled that the turbot is a kosher fish but their opinion is now generally rejected by British Orthodox Jews.
Worms, frogs, eels, and all shellfish such as crabs and prawns [shrimp] are not kosher. With regard to locusts, the Bible (Leviticus 11:21-2) does state that four species of locust are kosher, but it is difficult to know how these can actually be identified, so that nowadays very few observant Jews eat locusts (although in some oriental countries the kosher type of locusts are eaten, as they were in the biblical period).
As noted elsewhere, the Bible forbids the eating of the meat of an animal torn (terefah) by wild beasts and it also forbids (Deuteronomy 14:21) the meat of an animal that has died of its own accord, called nevelah, a carcass. The Rabbinic understanding of these two terms is that any animal that has not been killed in the manner known as shechitah [kosher slaughtering] istreated as nevelah, and any animal that has serious defects in its vital organs is treated as a terefah, so that its meat is forbidden even if it has been killed in the proper manner. This applies to birds as well as to animals.
There is a vast literature on how to determine which type of organic disease renders an animal or bird terefah. Observant Jews, for instance, will bring to a rabbi a chicken that seems to have some defect when it is opened up. After an examination, the rabbi will declare it to be either kosher or terefah. Similarly, after an animal has been killed, the shochet, the one who performs the act of shechitah, isrequired to carefully examine the lungs of the animal to see whether there are adhesions, some of which render the animal terefah.
Not all adhesions on the lungs render the animal terefah, and a rabbi is called upon to decide in doubtful cases. But the practice has developed among the more observant of permitting only animals the lungs of which have no adhesions at all. Such an animal is called glatt kosher, from the Yiddish "glatt" meaning smooth i.e., the lungs are smooth, without adhesions. A curious development from this in more recent years is to extend the term "glatt kosher" to all products, so that when a product is stated to be ["glatt"] the meaning is: free of any possible taint that can render it terefah. "Glatt kosher" has thus come to mean something like "very kosher" or "strictly kosher."
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