1) Animal anatomy 
A typhlosole is an internal fold of the intestine or intestine inner wall. 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 surfa...
• (n.) A fold of the wall which projects into the cavity of the intestine in bivalve mollusks, certain annelids, starfishes, and some other animals.
(in annelids and many bivalve mollusks) an infolding along the inner wall of the intestine.