Paramyosin
Paramyosin is a large, 93-115kDa muscle protein that has been described in a number of diverse invertebrate phyla. Invertebrate thick filaments are thought to be composed of an inner paramyosin core surrounded by myosin. The myosin interacts with actin, resulting in fibre contraction. Paramyosin is found in many different invertebrate species, for example, Brachiopoda, Sipunculidea, Nematoda, Annelida, Mollusca, Arachnida, and Insecta. Paramyosin is responsible for the "catch" mechanism that enables sustained contraction of muscles with very little energy expenditure, such that a clam can remain closed for extended periods.
Read more about this topic: Myosin