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1. We describe the phasic neuromuscular system of the crayfish telson and establish its homology with the abdominal flexor system that provides the powerstroke for tailflip escape responses. 2. Three paired phasic telson muscles are innervated by 11 paired neurones which have somata in the terminal (sixth) ganglion and axons in the sixth nerve. These are the posterior and ventral telson flexors and the anterior telson muscle. 3. Studies of embryonic ganglia provide evidence that the sixth ganglion is a fusion product of two ancestral ganglia, plus a partial ganglion that is not homologous with the segmental ganglia. 4. Two of the telson flexor motor neurones are homologues of the single motor giant found in each anterior hemiganglion. Among the shared features which led to this conclusion are: size, soma position, distribution of terminals to the muscles, dendrite morphology, pattern of direct inputs from the giant axons, and the marked tendency for low-frequency depression of the neuromuscular synapse. 5. Two of the telson flexor neurones are homologues of the single flexor inhibitor found in each anterior hemiganglion. In addition to numerous morphological similarities, these two cells produce IPSPs in the telson flexor muscles. 6. Six of the seven remaining motor neurones were identified as homologues of the non-giant fast flexor excitors of anterior ganglia. These can be divided into two uneven groups according to their ganglia of origin. The sixth segmental group of fast flexor motor neurones consists of four neurones (one less than expected) and the seventh segmental group consists of two neurones (three less than expected). 7. The remaining neurone provides the sole innervation of the anterior telson muscle. Although previously classified as a telson flexor muscle, we found that the anterior telson muscle moves the uropod but not the telson. The innervation of this muscle and the pattern of inputs to the anterior telson motor neurone from identified interneurones are unlike that of any fast flexor muscle or motor neurone. We conclude that the anterior telson muscle and its motor neurone are not homologues of anterior components of the fast flexor system. 8. In anterior ganglia, a prominent premotor neurone known as the segmental giant is presynaptic to all fast flexor motor neurones except the motor giants and flexor inhibitors. We identified a single paired cell in the sixth ganglion which appears to be the segmental giant homologue.

The Telson Flexor Neuromuscular System of the Crayfish: I. Homology with the Fast Flexor System