and what about the giant arthropleurid myriapods of the late Paleozoic

Indeed, a large living millipede is a virtual walking ecosystem of gut-associated organisms. Attached to the chitinous gut lining of living millipedes are filamentous actinomycete bacteria, protoctists, and even nematodes -- all infesting the myriapod's body while it thrives in perfect health. Furthermore, there is a major class of fungi, the Trichomycetes ("hair fungi") which are found only within the guts of these myriapods and of certain other organisms such as centipedes. Trichomycetes of the genus Enterobryus attach to the millipede gut lining by means of a suckerlike holdfast. The holdfast itself is often associated with radiating clusters of filamentous actinomycete bacteria. It is fair to speculate that the giant arthropleurid myriapods of the late Paleozoic may have had, in addition to trichomycetes, an even more complex and interesting gut biota because their guts were so large.

McMenamin and McMenamin, Hypersea, p. 199.