Typhlosole

[1] Typhlosoles occur in bivalve mollusks, lampreys and some annelids and echinoderms.

In earthworms, it is a dorsal flap of the intestine that runs along most of its length, effectively forming a tube within a tube, and increasing the absorption area by that of its inner surface.

Its function is to increase intestine surface area for more efficient absorption of digested nutrients.

In different earthworm families, the typhlosole appears to have multiple origins.

In shipworms, the typhlosole is the organ where the lignin in wood are digested by symbiont bacteria of the "Alteromonas-like sub-group".