Job 41:7

Second, he shows that man cannot use Leviathan like he uses fish which have been caught. Fishermen fill large nets with these fish with the larger specimens, and he expresses this saying, "Will you fill the nets with his skin?" He clearly says "skin" perhaps …

Wilt thou fill nets with his skin, and the cabin of fishes with his head? 34. What is designated by 'nets,' or a 'cabin of fishes,' except the churches of the faithful which make one Catholic Church? Whence it is written in the Gospel, The kingdom of heaven is like unto a net cast into the sea, and gathering of every kind of fishes. [Matt. 13, 47] The Church is in truth called the kingdom of heaven, for while the Lord exalts her conduct to things above, she already reigns herself in the Lord by heavenly conversation. And it is also rightly compared to a net cast into the sea, gathering of every kind of fishes; because when cast into this gentile world, it rejected no one, but caught the wicked with the good, the proud with the humble, the angry with the gentle, and the foolish with the wise. But by the 'skin' of this Leviathan we understand the foolish, and by his 'head,' the wise ones of his body. Or certainly by the 'skin,' which is outermost, are designated those who serve him as inferiors in these meanest offices, but by the 'head' those placed over them. And the Lord observing the proper order rightly declares that He will fill these 'nets,' or 'cabin of fishes,' that is, His Church, and the wishes of the faithful with his 'skin' first, and afterwards with his 'head.' Because, as we said before, He first chose the weak, that He might confound the strong afterwards. [l Cor. l, 27] He chose in truth the foolish things of the world, to confound the wise. For He gathered together the unlearned first, and philosophers afterwards; and He taught not fishermen by means of orators, but with wondrous power He subdued orators by means of fishermen. He says therefore, Wilt thou fill nets with his skin, or the cabin of fishes with his head? […]

Canst thou fill his skin with barbed irons? or his head with fish spears? This seems not so well to agree with the whale; whose skin, and the several parts of his body, are to be pierced with harpoons and lances, such as fishermen use in taking whales; and their flesh to be cut in pieces with their knives: but better with the crocodile, whose skin is so hard, and so closely set with scales, that it is impenetrable; See Gill on Eze 29:

He kept out of the reach of their instruments of slaughter, or, if they touched him, they could not touch him to the quick. That it was to no purpose to attempt it: The hope of taking him is in vain,

Canst thou fill his skin with barbed irons? - Referring to its thickness and impenetrability. A common method of taking fish is by the spear; but it is here said that the leviathan could not be caught in this manner. The common method of taking the crocodile now is by shooting him; see the notes at Job 41:1. Nothing is more remarkable in the crocodile than the thick and impenetrable skin with which it is covered; and the description here will agree better with this animal than with any other.