Halakhah for Chullin 132:28

That is that He commanded us to check for the signs of [permissible] fish, which are written [in the Torah]. And that is His, may He be blessed, saying, "These are what you may eat from all that is in the water" (Leviticus 11:8). And in explanation, they said in the Gemara (Chullin 66b), "One who eats an impure fish transgresses a positive and a negative commandment." As its statement - "These are what you may eat" - [makes] me understand that one besides these may not be eaten. And a negative commandment that is derived from a positive commandment is a positive commandment. Behold it has been made clear that its statement, "These are what you may eat," is a positive commandment. And the content of our saying that this is a positive commandment is what we have mentioned to you; and that is our being commanded to administer these signs, and to say that this is permissible to eat and this is not permissible to eat - as it is stated (Leviticus 20:25), "And you shall distinguish the clean beast from the unclean." And their distinguishment is though the signs. And therefore each and every one of these four types and their signs is a separate commandment - meaning the signs of the beasts and the animals; the birds; the locusts; and the fish. And we have already explained their expressions in each of the verses as an individual commandment. And the regulations of this commandment - meaning the signs of the fish - have already been explained in Chapter 3 of Tractate Chullin. (See Parashat Shemini; Mishneh Torah, Forbidden Foods 1.)

And so [too,] that which they, may their memory be blessed, said (Chullin 67b) [that] worms that are found in the intestines of fish are forbidden on account of [being] a swarming creature, since they come from the outside; but those found in them between the skin and the flesh or in the flesh are permitted. Those that are found in the intestines of the beast are forbidden, since they came from the outside. But even those found in the brain of the beast or in its flesh are forbidden, since there is not anything in the beast that is permitted without slaughter, and they are not susceptible to slaughter. And also since the Torah added them [by implication] to the forbidden. And [it is] like they, may their memory be blessed, said (Chullin 67b), "You shall abominate their carcasses' (Leviticus 11:11) - is to include deranin (the understanding of which is insects found between the skin and the flesh) that are in the beast." And if you will [ask], how is it that the embryo is permitted without slaughter; [it is] because the Torah permitted it - as we expound in the Gemara (Chullin 69a) from "among (which can also be read as, 'in') the beast[...] you may eat" (Leviticus 11:3). And so [too,] did they, may their memory be blessed, permit (Chullin 66b) water in vessels that became worm-ridden, or even [if they were] in ditches and caves - meaning to say, any gathered waters. And they said (Chullin 66b) that a man [may] swim and drink from them, so long as the insects have not separated to a different place. And even if they have separated to the sides of the vessel or the pit - once they returned to it, he may drink and not prevent himself [from drinking], as this is their habitat. And it appears that there is not even [an issue] of 'you shall not be disgusting' with them. And that is [the meaning] of their saying, "and he may drink and not prevent himself." And it appears that the reason is because they found explicit permissibility from Scripture for them - as they, may their memory be blessed, expounded (Chullin 66b), "'In the seas and in the streams' - [is it] that what has [signs], you may eat; what does not have signs, you may not eat. But in vessels, and similar to vessels, whether they have or whether they do not have, it is permitted."