The morphology of the Mauthner’s neuron and its afferent synaptic endings are comparatively investigated in Cyclostomata, Pisces and Amphibians with special reference to the axon cap. Two evolutive lines emerged: a Batracomorphic line (B line) and a Teleosteomorphic line (T line). Steps for the B line are: Acipenseriformes, Sarcopterigyi, Urodela and larval Anura; for the T line: Brachiopterigyi, Elopomorpha, Notopteridae, Clupeomorpha, Mormiridae, Ostariophysi, Acanthopterigyi. The main morphological features of the B line are: very large soma (up to $600 \mu m$ max. diam.), very numerous dendrites at lateral, ventral and medial sides, “simple” axon cap without any peripheral bush (canestro), very thick axon, no very specialized synaptic endings; for the T line they are: soma size up to $300 \mu m$ max. diam., very elongated and arcuate cell body, few branches on single lateral and medial dendrites, presence of cap dendrites, few and slender ventral dendrites, “composite” axon cap (central core and peripheral part) limited by glial cap cells from a very big peripheral bush or canestro, axon not so thick as in the B line but much better myelinated. An electrophysiological analysis of the B line axon cap is necessary. The peripheral bush appears to be not only an assemblement of swollen pre-axon cap fibres, but a real synaptic field. Some observations on the Mauthner cell regression related to its significance as a starter of the tail movement are presented.