Having been spread throughout the galaxy by freighters and cargo ships, duracrete slugs were large silica parasite mollusks that dug tunnels into building foundations and fed off the duracrete therein, although indigestible concrete was also eaten. This was then excreted on to the skin, forming armor. Duracrete slugs were virtually blind, but had feelers capable of sensing rock layers. While their average length was three to five meters, some specimens on Coruscant were known to reach ten meters in length, especially in the Coruscant underlevels.