Nothing to add to my esteemed colleagues, Lyonel. “Like a marine lung” is what it literally says; and the different relevant senses of “lung” (πλεύμων) that apply here, from Liddell-Scott, are:
- sea-lungs = jelly fish
The “jelly fish” meaning of πλεύμων is attributed to Plato, Aristotle, Theophrastus, and—surprise—Pytheas, as cited in Polybius. i.e. Your passage.
For jollies, I’m gonna try out my Mad Ancient Greek Skillz on the passage, without looking at the translation.
In describing the geography of Europe, Polybius says you should ignore the most ancient sources, and of those who scrutinised them, you should examine Dicaearchus, Eratosthenes (in this treatise on Geography), and Pytheas. Pytheas misled many, claiming to have gone around the entire expanse of Britain, and attributing a circumference to the island of more than 400,000. He further added in a story about the regions around Thule, where there was neither land nor sea nor air, but some sort of mixture of the three like a jelly-fish. He says that land and sea and everything float in this mixture, and that the mixture is like the stuff binding it all t0gether, being neither walkable not navigable. He says that he saw the jelly-fish-like mixture with his own eyes, and everything else he heard reported. That’s what is in Pytheas; and on his return he claims to have gone past the whole coast of Europe, from Cadiz to the Don.