Contractility and its Control in Peritrich Ciliates*†

SYNOPSIS A review of the recent literature on the structure and physico‐chemical properties of the myoneme and its specialization, the spasmoneme, of peritrich ciliates was made. Myonemes are composed of tightly packed bundles of 3–5 nm microfibrils which parallel, more or less, the long axis of the bundle and are of indefinite length. The presence of contractility in these ciliates is correlated with the presence of myonemes. Associated with the microfibrils is a system of membrane‐bound tubules and saccules continuous with the endoplasmic reticulum (ER). This system is known to accumulate calcium. Myonemes differ from muscles in their structure, solubility properties, birefringence pattern, and in the time during the contraction‐relaxation cycle at which they require energy. They may be related more closely to the cytochalasin B‐sensitive microfibrils of higher organisms than to muscles. Contraction of extracted stalks can be induced solely by raising the calcium ion concentration above a certain threshold. Thus the calcium‐accumulating myoneme‐associated ER would appear to play an important role in the control of myoneme contractility. A specialization at the interface between the myoneme and the ER membranes is described as it appears in Vorticella and Opercularia. This structure, called a linkage complex, is found both in the body myonemes and the spasmoneme and links the membranes of the ER to the microfibrils. It also has microfilaments that pass from the ER‐myoneme interface to the surface membranes. The uniqueness of this structure and its location suggests that it may play a role in controlling the movements of calcium between the ER and myoneme and also in transmission of messages from the pellicular membranes, possibly the alveolar sacs, to the ER.