The Structure and Operation of the Obliquely Striated Supercontractile Somatic Muscles in Nematodes.

Abstract

The structure of nematode somatic muscles is reviewed. They are obliquely striated and, therefore, supercontractile because they change in length to an exceptional extent by both myofilament interdigitation and shearing. A model is developed which shows that shearing is generated by interdigitation under specified structural conditions. It is concluded that the ability to supercontract is functionally associated with a hydrostatic skeleton, because obliquely striated muscles occur in a wide range of unrelated Phyla and a functionally identical muscle has evolved from cross-striated muscle in other animals with a hydrostatic skeleton, particularly certain insect larvae. Two, possibly 3, morphologically distinct groups of muscles exist in the Nematoda which correspond to the Classes Enoplea, Rhabditea and Chromadorea. Parallel sequences of change occur in the first 2, in which the contractile layer increases in area, the sarcomeres become smaller, and the Z-system more regular. Descriptive terms are redefined, thus: platymyarian, quasi-platymyarian, shallow coelomyarian, and coelomyarian occur in Rhabditea; cubomyarian, regular cubomyarian, and stratimyarian occur in Enoplea; and a characteristic chromadorid-circomyarian is known from 2 Chromadorea. The structure of the muscles corresponds well to the higher nematode classification, except in the Order Spirurida which may have arisen from the Enoplea and not the Rhabditea as generally accepted.