SYXOPSIS.A study of the 'longitudinal fibrillar bundle' (LFB) and the 'contractile fibrillar system' (CFS) of a large protozoan ciliate, Spirostomum ambiguunt, has been performed by means of an electron microscope. .4 system of sub-pellicutar fibrils has been newly found and its function is discussed. Each LFB runs parallel with a longitudinal row of ciliary bases. I t seems to be identical with the so-called kinetodesma. I t is composed of tubular fibrils arranged in layered sheets, each of which contains 13 to 3. 5 fibrils with the same diameter as the intra-ciliary fibrils and has a close connection to each EVERAL large ciliates that are capable of rapid S change of form in response to stimuli-Condylostoma, Stentor, Vorticella, Carchesium, Zoothamnium and others-have special cell organelles which are directly instrumental in their body contraction. The fibrous structures of these organelles have been studied since nearly a century ago, and since Lieberkuhn ( 3 ) found 'Muskelfasern' in the ectoplasm of Stentor, the corresponding structures have been recognized in several contractile ciliates and named mvoneme, myophane, neurophane, neuroid and the like. At present, the term myoneme seems to be generally and widely used for the structure considered as a contractile element, but it is still doubtful whether or not such fibrous structures actually contract. As far as we know, the so-called myoneme of Spirostomum

SYNOPSIS. A morphological study on the ectoplasm and the proboscis in the ciliate Didinium nasutum, has been performed by means of an electron microscope. The ectoplasm and the endoplasm of Didinium are separated by a fibrous layer. In addition to the ciliary apparatus and the filament system, the ectoplasm is characterized by having ectoplasmic vacuoles enclosing cross‐striated bodies and by having small rods surrounding the ciliary basal body. The filament system is composed of 4 types of tubular filaments: primary filaments originating from the basal body, secondary ones coursing longitudinally along the cell periphery, tertiary ones going down in cylindrical arrays from the periphery of the proboscis into the endoplasm, and finally kinetosomal ones from the base of the basal body into the endoplasm through the newly found pore of the fibrous layer. The fine morphology of the trichites in the proboscis is elucidated three‐dimensionally and illustrated schematically. Moreover, the correlation among the small rod, ectoplasmic vacuole and trichite is discussed.

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