typhlosole

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

A typhlosole is an internal fold of the intestine or intestine inner wall.

Sparganophilus species are characterised by a simple gut with no gizzard and no typhlosole.

The Criodrilidae are characterised by holoic nephridia absent from anterior segments (cf. Pontodrilus), a simple gut with no gizzard and no typhlosole.

The Lumbricidae, for example, have a typhlosole which is an infolding of all layers of the intestine wall, whereas in some other families (e.g. Megascolecidae), it is an infolding of only the inner layer, and in many earthworms it is absent.