The relationship existing between temperature and the frequency of the heart beat has been studied for' a number of animals (Crozier and Federighi, '26'25 ; Crozier and Stier, '26'25 b ; Crozier, '25'26 b ; Glaser, '25'26 ; Fries, '26'27 ; Crozier and Stier, '26'27, for complete bibliographies), but excepting for Rogers' ('11) work on Nereis virens (fig. 1) and on Tubifex, which, because of the long intervals between temperatures, cannot be adequately analyzed, annelids have been little investigated from this standpoint. They are especially favorable objects, as many of them possess a dorsal 'heart' that is clearly visible and easily observed. The following data on the relation of temperature to 'the frequency of heart beat in Nereis were collected at Harvard University during the winter of 1925 in connection with some histological and physiological studies on the blood vessels of annelids (,Federighi, '28) and have been supplemented by observations made at the United States Fisheries Biological Station, Beaufort, North Carolina, in June, 1929. Although it is true that values of the temperature characteristic cannot be employed as diagnostic for nervous, myogenic, or other specific modes of control, one is permitted to classify them and to point out consistencies (Crozier, '25'26 b; Crozier and Stier, '26'27). Thus the values of the temperature characteristics for the rate of heart beat in Nereis virens agree with those reported for certain other 393