There are difficulties in making accurate predictions of function from structure. Sometimes one structure has many functions or many structures serve the same function.
The nervous system is also important with regard to function, and we can not access the motor programs the nervous system is capable of. Changes in the nervous system create dramatic differences in functions of morphological features. Alterations in output can alter function of structures and their effect on organismal performance, or can alter the behavioural pattern of the whole organism.