Ultrastructural study of two kinds of muscle in sea anemones: The existence of fast and slow muscles.

The sea anemones studied have two morphological types of muscle fiber. Types A and B are distinguishable on the basis of myofilament patterns, size of fibers, responses to fixation, and staining with methylene blue. Observation of the muscle in both resting and contracted states has shown that the two types do not result from differences in contraction state of the muscle. The fine structural characteristics distinguishing A and B fibers are similar to those which distinguish fast and slow muscle fibers in higher animals. The distribution of A and B fibers in Stomphia and Aiptasia is consistent with the distribution of fast and slow muscles in these two species. It is proposed that the A and B fibers represent two morphologically distinct kinds of smooth muscle, and that the capacity for fast and slow contraction in the muscles of Stomphia and Aiptasia, and possibly in all actinians, is due to morphological differentiation in the muscle system.