First observation of a muscle spindle in fish.

In many groups of vertebrates, the muscle spindle is a specialized sensory organ for the detection of muscle stretching. The structure of the spindle varies among vertebrate classes. Moreover, Barker has asserted that Amphibia are the most primitive vertebrates to possess muscle spindles. Extensive studies, made mainly on the locomotor myotome, seem to show that the muscle receptors of fish are less specialized than those of more advanced animals, and that muscle spindles are absent. However, little attention has been paid to the jaw-closing muscle. We report here our finding of a very simple muscle spindle with a single intrafusal fibre in the well-developed jaw-closing muscle, adductor mandibulae, in a primitive teleostean, Oncorhynchus masou (Brevoort).