The neural basis of escape swimming behaviour in the squat lobster Galathea strigosa. I. Absence of cord giant axons and anatomy of motor neurones involved in swimming

1. The anatomy and physiology of neurones underlying escape swimming behaviour in the squat lobster, Galathea strigosa, have been investigated, and the results are discussed in the context of the evolution of decapod escape behaviour. 2. In contrast to crayfish, hermit crabs and a number of other related decapods, Galathea does not possess a giant fibre system for escape. 3. Fast flexor motor neurones (FFs) and fast extensor motor neurones (FEs) have been shown, by cobalt backfilling, to be homologous with crayfish FFs and FEs in number, size and distribution of somata. A small degree of intersegmental and interspecific variation is noted. The flexor inhibitor (FI) neurone is described in terms of its central anatomy, peripheral function and peripheral branching pattern. In each of these respects the neurone is found to be homologous with the crayfish FI. 5. The neurone homologous with the crayfish motor giant (MoG) in its soma size and position is found to be a typical FF in Galathea. This 'MoGH' contrasts with the crayfish MoG in having central neuropilar arborization and in lacking axonal branches in the connectives. These differences can be accounted for by the absence of cord giant axons.