The frequency of reversal of the respiratory current is recorded for Cancer pagurus, Macropipus puber, Corystes cassivelaunus and Homarus gammarus by cannulating the gill chamber and recording pressure changes therein. The predominant respiratory current flows backwards in Corystes and forwards in the other three, but all show rhythmic reversals of that current which appear to form an integral part of the mechanism of gill ventilation. Reversals appear to have significant gill cleaning functions only in Homarus andperhaps also Corystes.
Measurements of gill ventilation volumes support these findings. Macropipus puber takes in almost all respiratory water at the bases of the chelae, anterior to the gills, and it shows a high reversal frequency apparently to irrigate the posterodorsal aspect of the gills. Cancer pagurus takes in a considerable volume of respiratory water at the bases of the last pair of peraeopods in addition to that entering at the bases of the chelae. This species shows fewer reversals, since the posterior gills are irrigated during normal forward flow, but artificial closure of the posterior openings results in an increased reversal rate.
Comparisons are made of the gill ventilation patterns of all these species, together with Carcinus maenas described previously, and their functional inter-relationships are discussed.