The Structural Basis of an Innate

The abdominal nervous system of the crayfish contains six serially homologous ganglia, each containing approximately 650 neurones. No two ganglia are identical, and the ganglia interact extensively. Studies confined to intraganglionic interactions thus yield limited and sometimes misleading information. Each ganglion contains intrinsic (local) interneurones, motor neurones and projecting interneurones in roughly equal numbers, except in the specialized terminal ganglion where the ratio of these cells is approximately 3:2: 1. Although the number of nerve cell bodies in a ganglion is small enough to be tractable, integration occurs in the neuropile, which contains terminals from interneurones and afferents that outnumber the neurones originating in the ganglion by at least ten to one. The abdominal nervous system responds almost exclusively to a variety of mechanosensory stimuli. It has very limited light sensitivity. Other modalities, notably chemosensitivity, are undescribed and may be lacking. The effectors of the abdomen consist of fast axial muscles (used for tailflip-powered escape), slow axial muscles (for setting abdominal posture), appendage muscles (for swimmeret beating), and slow muscles of the intestine and rectum (that control gut emptying). The fast and slow muscles of the tailfan are specialized homologues of the axial and appendage muscles.