Alternative pathways in the development of the clausilial apparatus in shells of Albinaria and Isabellaria (Gastropoda, Pulmonata, Clausiliidae)

In Albinaria and Isabellaria the step-wise development of a G-type instead of an N-type clausilial apparatus starts with the formation of a lamella fulcrans instead of a lamella spiralis, shortly after the initial formation of the lamella columellaris. The other lamellae as well as the fullgrown clausilium are formed later on. It is hypothesized that the genetic background of such an early shift in the developmental programme might be relatively simple, facilitating both parallel evolution and introgression.