The video gives clear evidence in a camouflaged land snail that the extraneous layer is positioned on the shell by the animal itself. In contrast to previous hypotheses it is an active process.
The snail species Napaeus barquini from the Canary island of La Gomera occurs on rock faces covered with crustose lichens.
This snail grazes lichen material from the substrate and applies it to the surface of its shell with its mouth.
The snail is able to deposit lichen material even in the farthest regions of its shell by extending its body amazingly far beyond the shell margin.
As a result, a snail carries mountains of bizarre protuberances on its shell.
These results were published in the scientific journal Zoological Science (24: 869-876 (2007) doi: 10.2108/zsj.24.869, bioone.org/doi/pdf/10.2108/zsj.24.869)