smooth muscle tissue (...) the first bilateral animals possessed only smooth muscles with the molecular repertoire necessary to build a striated muscle. (...) it is more parsimonious to regard striated muscle cells as a sister cell type to the smooth muscle cells. In this scenario, striated and smooth muscles would have arisen in the stem lineage that led to the Nephrozoa (i.e. all Bilateria exclusive the acoelomorphs) (Hejnol et al., 2009), from an 'acoel-like' smooth muscle, by segregation and divergence of functions and through differential recruitment of additional genes[well established][VHOG] A non-striated muscle that is composed of spindle-shaped cells. Smooth muscle usually is organized into sheets that line cavitated organs.[TAO] AAO:0010244 AEO:0000141 BTO:0001260 CALOHA:TS-0943 EFO:0000889 EHDAA2:0003141 EMAPA:32717 EV:0100378 FBbt:00003525 FMA:14070 GAID:167 GO uses visceral and smooth interchangeably. However visceral can also be used in the sense of the viscera. Many fly annotations to smooth muscle terms. If we want to be inclusive of insects have to have a general definition of tissue that includes cells. Involuntary muscle tissue consisting of uninucleate spindle-shaped fibers.[AAO] MA:0000166 MAT:0000303 MESH:D009130 MIAA:0000303 Muscle tissue which is unstriated, composed primarily of smooth muscle fibers surrounded by a reticulum of collagen and elastic fibers. Smooth muscle differs from striated muscle in the much higher actin/myosin ratio, the absence of conspicuous sarcomeres and the ability to contract to a much smaller fraction of its resting length[GO]. OpenCyc:Mx4rvvSS3pwpEbGdrcN5Y29ycA TAO:0005274 UBERON:0001135 UMLS:C1267092 VHOG:0001246 WBbt:0005781 XAO:0000175 ZFA:0005274 galen:SmoothMuscle galen:SmoothMuscleTissue involuntary muscle non-striated muscle smooth muscle textus muscularis levis; textus muscularis nonstriatus textus muscularis nonstriatus uberon visceral muscle visceral muscle tissue muscle tissue