Abstract: Euglenids are unicellular aquatic organisms capable of moving either by beating a flagellum or by executing dramatic shape changes. These are accomplished thanks to a complex structure underlying the plasma membrane, made of interlocking proteinaceous strips, microtubules, and motor proteins. We study the mechanisms by which the sliding of pellicle strips leads to shape control and locomotion, by means of both theory (through the mechanics of active surfaces and its coupling to computational fluid dynamics for the surrounding fluid) and experiments. Moreover, we implement them into a new concept of surfaces with programmable shape, obtained by assembling 3d-printed strips in a construct mimicking the biological template. We show that the subtle balance between constraints and flexibility leads to a wide variety of shapes that can be obtained with relatively simple controls. This suggests that euglenids exploit the passive resistance of body parts to reduce the complexity of controlling their shape.