1. A characteristic behaviour, the ‘Mauthner-initiated startle response’, was recorded and quantitatively analysed with high-speed cinematography (200 frames/sec) after vibrational stimulation in 11 of 13 teleost species which possess Mauthner cells. 2. The latency of the response is 5–10 msec. This behaviour has: (a) an initial phase, the ‘fast-body-bend’, lasting about 20 msec and consisting of a stereotyped displacement of the head and tail to one side and (b), a second phase, the ‘return-flip’, consisting of a non-stereotyped flip of the tail to the opposite side. 3. Within 100 msec after the start of the Mauthner-initiated startle response, most fish were displaced 0-5-1-5 body lengths from their initial position. The variability of the animal's location after 100 msec suggests that the behaviour is adaptively non-predictable; 4. In goldfish, the Mauthner-initiated startle response could also be elicited by visual stimulation. 5. We conclude that the fast-body-bend is the direct result of activation of one Mauthner cell and its spinal motor neurone pool. 6. In four species we described examples of apparently non-Mauthner initiated startle responses.

To determine whether the largely ipsilateral, inverted representation of mouth parts in the ventrobasal thalamus of sheep was unique to that species or an expansion of a general mammalian pattern, the corresponding thalamic projections were mapped electrophysiologically in a selected series of mammals (oppossums, agoutis, squirrel monkeys, cats, raccoons, and sheep) representing major branches of evolution among therian mammals. In mapping, tungsten microelectrodes were used to record multi-unit discharges in the thalamus in response to mechanical stimulation of oral surfaces. The pattern of projections seen in sheep is not a general mammalian pattern; there is extensive variability among mammals in the laterality and internal orgainzation of the projections from the mouth. In spite of the great variability, the results suggest an hypothesis concerning phylogenetic trends: descendants of palaeoryctoid insectivores (cats, raccoons, and sheep in our sample) have extensive ipsilateral projections from the mouth, in other therian mammals (opossums, agoutis, and squirrel monkeys in our sample) the ipsilateral component is small or absent.

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