The muscuar system is present in animals of the phylums Mollusca, Arthropoda and Annelida. Annelida are all very similar organisms.
Sources:
http://en.wikipedia.org/wiki/Land_snail
http://www.ncbi.nlm.nih.gov/pubmed/18165243
http://www.ehow.com/how-does_4744976_a-clam-move.html
http://en.wikipedia.org/wiki/Insect_physiology
http://en.wikipedia.org/wiki/Spider
http://www.britannica.com/EBchecked/topic/529352/scorpion/47766/Internal-features
http://www.britannica.com/EBchecked/topic/398553/muscle/58906/Muscle-in-soft-animals
http://www.ncbi.nlm.nih.gov/pubmed/1151280
http://en.wikipedia.org/wiki/Land_snail
http://www.ncbi.nlm.nih.gov/pubmed/18165243
http://www.ehow.com/how-does_4744976_a-clam-move.html
http://en.wikipedia.org/wiki/Insect_physiology
http://en.wikipedia.org/wiki/Spider
http://www.britannica.com/EBchecked/topic/529352/scorpion/47766/Internal-features
http://www.britannica.com/EBchecked/topic/398553/muscle/58906/Muscle-in-soft-animals
http://www.ncbi.nlm.nih.gov/pubmed/1151280