body wall muscle sarcomere morphology variant M line variant Animals exhibit variations in the form, structure or composition of a planar structure within the muscle sarcomere which lies midway within the thick filament (A) band, attaching to the plasma membrane at its base and running up towards the inward surface of the myofilament lattice, compared to control animals (Wormatlas). WBPhenotype:0001891 C_elegans_phenotype_ontology