Yes, the given statement is true that combination of nerve and muscle fibers is fundamental to most animals because this combination helps in the voluntary movements.

Whenever we want to do any movement by our choice command is given to the skeletal muscles which are responsible for the voluntary movements by the nervous system. If nerve cells and muscles will lack coordination in between them then we have nothing like voluntary movements.

So, all animals which show voluntary movements have a functional combination of nerve and muscle tissue and is fundamental to most animals.