Chiton mollusks can see with eyes embedded in their shells

The chiton is a type of mollusk, and the species that was studied is named  Acanthopleura granula. It
was found that these animals have lots of small lenses with mineral bases that are scattered
across their armored shells. Each is smaller than a tenth of a millimeter across.

The eyes allow the animals to see what is happening in their nearby environment without having
to open a shell or anything like that. In effect they are seeing through the shell. The rest of
the shell is opaque, and only the eyes are transparent.

A team at the at the Argonne National Laboratory,  performed high-resolution X-ray tomagraphy
tests on individual eyes in the chiton shell, in order to confirm that the lenses can form images. The chitons can rapidly respond when predators approach. They do so by gripping onto the substrate below them and not allowing themselves to be dislodged from it.

There is a downside to this. The study team also learned that as chiton eyes increase in size
and complexity, the armor actually performs less effectively as the eyes weaken it somewhat.