The Fine Structure of an Elementary Contractile System

Abstract

The myoid fibrils composing the contractile system of Vorticella (protozoon) are made up of fine elemental filaments each 30 to 40 A in width. The fibrillar bundles are in a close relationship with a membrane limited canalicular system that resides both inside the bundles and around them. This system is connected with the endoplasmic reticulum (ER) that exists in the cytoplasm. The cytoplasmic myonemes join at the origin of the stalk to form its axial bundle. At the same place, both the canaliculi of the axial bundle and the canaliculi of the cytoplasmic myonemes are connected. The relationship between canaliculi and myofilaments of Vorticella is similar to that existing between muscle fibrils and ER in muscle cells of higher animals.