One of the lobsters kept in the tanks of the Plymouth Laboratory moulted on the night of 2 0 August 1940. This opportunity was taken to examine the changes in water content and osmotic pressure of the freshly moulted as compared with unmoulted lobsters. Though only one recently moulted individual has been examined, the results seem to be worthy of note since so far as we are aware no data are available on the subject in regard to Homarus. The biology of moulting among lobsters has received the attention of many investigators (vide Herrick (1895) and Drach (1939) for summary and literature).

The water content was determined by distilling the fresh lobster together with a known weight of sea water under xylol. This method is a modification of that of Dean and Stark which one of us (A. G. L.) has tried on a number of animals and found to give accurate results. The density was estimated by a modification of a method previously described (Lowndes, 1938). Osmotic pressure of blood and the external medium were measured by Baldes's (1934) modification of the Hill thermoelectric technique, as employed by one of us (N. K. P.) in the study of prawns and other crustaceans. The water content of the moulted skin was calculated from its dry weight taken after dehydrating in a hot air oven for 48 hr. at 105° C. Osmotic pressure was measured 17±6 hr. after moult; density and water content after 34±6hr. The temperature of sea water at which the density was estimated was 17° C.