A MAP OF DISTAL LEG MOTOR-NEURONS IN THE THORACIC GANGLIA OF 4 DECAPOD CRUSTACEAN SPECIES

We describe the numbers, central positions, and axonal exit routes of the distal leg motor neurons of four decapod species: squat lobsters ( Munida quadrispina), spiny sand crabs (Blepharipoda occidentalis), mol e sand crabs (Emerita analoga), and signal crayfish (Pacifastacus leni usculus). As predicted by previous physiological and anatomical identi fication of axons at the periphery in crayfish and lobsters, cobalt ba ckfills reveal about seventeen cell bodies, which are found in four ar eas in the ganglion. By comparing their positions and neurite morpholo gies with the previously identified neurons, functional identification s could be assigned to most of them. The common inhibitor and stretche r inhibitor are located posterior-medial, An anterior-lateral cluster of about twelve somata includes the opener=stretcher excitor, one of t wo bender exciters (bender exciter ex), four flexor exciters, and two exciters each to the extensor, reductor, and closer muscles. Three cel l bodies are posterior-lateral. Of these, the opener inhibitor and the second bender exciter (bender exciter beta) are on about the same dor soventral plane. The third posterior-lateral cell, the accessory flexo r exciter, is noticeably more dorsal than the other two posterior-late ral cell bodies. The reductor muscle is innervated by at least three n eurons: the putative common inhibitor and fast and slow exciters. None of the leg motor neurons project into the contralateral hemiganglion. The most variable feature across species is the nerve through which m otor axons exit the ganglion: axons leave the ganglia via different ro utes in each of the four species examined. These differences in the ax ons' pathway, however, are insufficient to explain the differences in motor output and behaviour of these four species.