LIGHT AND ELECTRON-MICROSCOPIC OBSERVATIONS IN RHYNCHOPUS-COSCINODISCIVORUS SPEC NOV, A COLORLESS, PHAGOTROPHIC EUGLENOZOON WITH CONCEALED FLAGELLA

Rhynchopus coscinodiscivorus sp. nov. was found in the North Sea near List/Sylt, feeding on the diatom Coscinodiscus concinnus. It is pear-s haped but highly variable in form and has an apical papilla, a subapic al flagellar pocket and a subapical cytostome. The two short flagella contain microtubules which are not regularly arranged in an axoneme. T he flagella remain largely concealed inside in the flagellar pocket, t he wall of which is supported by microtubules and a band of rods, the latter extending to form a ledge over the apical papilla and into the cytostome, causing its ''plicate'' appearance. The rods then accompany the anterior part of the cytopharynx which runs approximately paralle l to the axis of the cell, nearly normal to the flagella, and is, in a ddition, associated with microtubules and microfilaments and a lacuna along its anterior part. The cell apex is reinforced by peripheral mic rotubules. It contains the nucleus, some dictyosomes and most mitochon dria, while the food vacuoles and reserve granules occur mainly in the posterior part of the cell. The mitochondria have only very few invag inations of the inner envelope membrane. Trichocysts, muciferous bodie s, paramylum grains and a euglenoid pellicle with pellicular strips ar e absent. R. coscinodiscivorus moves with euglenoid (''metabolic'') co ntortions, with rapid, jerking contractions of the cell apex and with gliding motions. The movement is inhibited by cytochalasin D.