The difference in morphology of cocci and bacilli lies in shape of the bacterium. Cocci and bacilli differ in their shape. Cocci are spherical in shape: whereas bacilli are rod-shaped. Cocci are usually aflagellate, i.e. lacking flagella. Bacilli may be aflagellate or flagellate. The number and position of flagella differ in different bacillus forms. Both cocci and bacilli may occur singly or in aggregation of two or more and termed accordingly.