In most fish groups, a large pair of Mauthner neurons provides a distinctive and conspicuous pathway linking the motoneurons of the cord to nerve VIII input. Physiological studies have been almost entirely on the apparatus in teleosts (review by Diamond, 1971), but its morphology is essentially similar in other groups, and it seems likely that wherever found in gnathostomes, its function is the same: to mediate a startle response involving a single rapid tail flip. It is therefore, as Whiting (1957) has emphasized, an important part of the reflex system in those lower chordates in which the myotomal musculature is used in locomotion. Not surprisingly, where locomotion is carried out in a different way (as for example in adult anura), the Mauthner neurons of the larva degenerate and disappear, as they do in a variety of teleosts where a tail flip startle response is inappropriate in the adult.