composed_primarily_of organ muscle organ AAO:0011066 EMAPA:32715 EV:0100146 FMA:5022 GAID:131 It seems clear that the metazoan ancestor inherited from its unicellular descendants an actin cytoskeleton and motor-proteins of the myosin superfamily. Within metazoans, these two molecules were arranged into effective contractile units, the muscles. The basic trends for muscle evolution are already expressed in the diploblastic taxa.[well established][VHOG] MA:0000015 OpenCyc:Mx4rv2kf-5wpEbGdrcN5Y29ycA Organ consisting of a tissue made up of various elongated cells that are specialized to contract and thus to produce movement and mechanical work[GO]. Portion of tissue composed of contractile fibers.[TAO] TAO:0005145 VHOG:0001245 XAO:0000172 ZFA:0005145 galen:Muscle muscle muscle tissue