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

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