Postembryonic development of dorsoventral and longitudinal musculature in Pycnophyes kielensis (Kinorhyncha, Homalorhagida).

We investigated the development of dorsoventral and longitudinal musculature in all postembryonic stages of the kinorhynch Pycnophyes kielensis. Although the earliest stages have only 8 externally separated trunk segments, they already possess dorsoventral muscles for 10 (prospective) trunk segments. The last, 11th, pair is added in the third juvenile stage. Longitudinal musculature, in contrast, is slower to develop and reaches its full length only in the adult. In several juvenile individuals, single fibers project from the longitudinal musculature into the following segments. In all juvenile stages, longitudinal muscles are continuous between segments, whereas in adults they are segmentally separated from each other. Such late occurrence of a segmental pattern in the longitudinal musculature is in contrast to patterns of muscle development in arthropods and annelids.