The Co-ordination of Swimmeret Movements in the Crayfish, Procambarus

Many Crustacea are characterized by the use of their appendages for swimming and feeding. In the lower Crustacea both the thoracic and abdominal appendages move for these purposes in a typical metachronal rhythm. The similar rhythmical movements found in the abdominal appendages of the crayfish (Hughes & Hickey, in preparation) are most probably based on similar nervous mechanisms, but here their function is not so clear. It is probably connected with the increased need for aeration when the animal burrows in mud and, in the female when in ' berry', with the oxygen supply for the developing larvae. The swimmeret movements provide very good material for a study of the general problem of the mechanisms of nervous control of rhythmic limb movements. Few observations have been made on Crustacea from this point of view and the crayfish is very suited for this purpose as considerable information is now available concerning its nervous system (Hughes & Wiersma, i960; Wiersma, 1958; Wiersma & Hughes, 1961). The preparations also proved of interest as in some the influence of stimulating fibres descending from the brain was shown. Clear evidence for the presence of pathways with excitatory or inhibitory effects on the discharge of motoneurones in the abdominal ganglia has been obtained.