[A new type of flagellar structure. Type 9+n]

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Abstract
The ultrastructural study of the Eoacanthocephala sperm cell shows a variation from 0-5 in the number of the axial fibers in the axoneme. All the species of the order Eoacanthocephala studied show this variation; moreover, every individual possesses simultaneously several different structural types. This is a new flagellar organization: 9 + n, with 0 .ltoreq. n .ltoreq. 5. In the Quadrigyridae and the Tenuisentidae families, n varies from 0-4, with a maximum of 2 for most individuals, exceptionally at 1 for some individuals. In the Neoechinorhynchidae family, n varies from 0-5 with a conspicuous prevalence of 3 (from 84-99%, according to the individual). These results prompted a reexamination of the 2 other orders of Acanthocephala in which the structural types 9 + 2 or 9 + 0 were considered as fixed. A few flagella, the structure of which is different from the prevalent one, were found. It seems that the number of the central fibers of the axoneme in the Acanthocephala sperm cell is never absolutely fixed.