Morphology and Morphogenesis of Onychodromus quadricornutus n. sp. (Ciliophora, Hypotrichida), an Extraordinarily Large Ciliate with Dorsal Horns1

Abstract
The morphology and the morphogenesis of the freshwater hypotrich ciliate Onychodromus quadricornutus n. sp. have been investigated using living organisms, protargol impregnation, and scanning electron microscopy. Some preliminary and supplementary results about the morphogenesis of O. grandis and Laurentiella acuminata are included. The new species is unique among all described hypotrichs in having four dorsal horns, whose function is unknown. In addition, O. quadricornutus is probably the most voluminous hypotrich ciliate known (2 times 10‐6‐5 times 106μm3). Its morphogenetic pattern resembles the oxytrichids O. grandis and L. acuminata. The strongest apomorphic character, which unites these three species, is probably the multiple fragmentation of the dorsal primordia during morphogenesis. This fragmentation causes the characteristic high number and more or less irregular distribution of the dorsal kineties in the non‐dividing individuals.

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