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1. The condition of the ovaries of intact and eyestalkless Uca pugilatorfrom Florida was examined throughout the period from mid-September to the end of May. The frequency of molt for the animals was also observed.2. Ovarian weights for intact crabs decreased from September to December and began to increase in January. The increase did not occur uniformly throughout the population, and by March all stages of maturation were present. First oviposition occurred in April, involving about 30% of the females. No oviposition occurred as late as mid-September.3. Crabs that oviposited in April did not exhibit further vitellogenesis prior to molt which occurred in late May. A high frequency of molting again occurred in intact crabs in November and December.4. Removal of eyestalks led to initiation of vitellogenesis in immature ovaries at all times tested and also appeared to increase the rate of vitellogenesis in already maturing ovaries.5. Removal of eyestalks increased the frequency of molt maximally in September and late April and caused no increase in November and December or for post-ovigers in late May.6. The pattern of reproduction and molt for this species in the months studied appears to consist of the following sequence: vitellogenesis-oviposition-incubation-hatching-molt. The breeding season could be extended by repetitions of the sequence or compressed by increasing the degree of synchronization.

1. O2-consumption of potatoes was recorded continuously through the fourmonth period, February-May, 1956.2. There were daily fluctuations in rate which, even using 3 x 7-hour moving means, displayed a mean amplitude of about 14%.3. Five independent respirometer-recording systems yielded mean daily cycles for the four-month period ranging in amplitude from 4.8 to 10.0% with a mean of 8.2%.4. The mean cycles were of two forms, one essentially 180° out of phase with the other.5. Solar-day and lunar-day mean fluctuations in background radiation were also determined for the period of the investigation.6. A small but very highly significant correlation existed between the fluctuations in amplitude of the daily cycles in radiation on day n-1 and amplitude of the daily fluctuations in O2-consumption in the potato on day n.

1. Eight species of living things, ranging from lower to higher plants and lower to higher animals, in studies over a three-year period and including approximately 850 species-days have exhibited without exception a statistically significant leadcorrelation on barometric pressure with an over-all mean coefficient of about 0.5.2. The correlation involved sometimes only the 5-6-7 A.M., sometimes only the 5-6-7 P.M., and other times the mean daily rates of O2-consumption.3. The sign of the correlation was sometimes positive and other times negative. Sign changes, when they occurred during a single period of study were abrupt, and correlated with a 180°-shift in the phase relationships of the concurrent mean solar-day cycles.4. In twelve periods of study, ranging from one to four months each, the correlation in nine cases centered on day n+ 2 of barometric pressure. In two cases it centered on day n+ 1 and in one, on day n+ 7.

1. The form of the mean diurnal rhythm of O2-consumption of Uca pugnaxis described and found to be practically identical for the summers of 1955, 1956, and 1957.2. The mean lunar-day rhythm of O2-consumption is described for the summers of the same three years. The curve for any one of these years is indistinguishable from that for either of the other two years.3. The mean lunar-day rhythm consists of two maxima, of equal magnitude, occurring approximately at lunar zenith and at lunar nadir; between the maxima are two minima symmetrical with respect to time of occurrence and magnitude.4. For the primary lunar rhythm the ratio of maximum to minimum is about 1.4; for the diurnal rhythm the ratio of maximum to minimum is 1.2 in 1955, 1.4 in 1956, and 1.2 in 1957.5. Because of the amplitude of the lunar component of the rhythm, the data for single days reveal clearly the progression of lunar maxima and minima.6. Because of the equality in amplitude of fluctuations correlated in time with lunar zenith and with lunar nadir, the overt rhythm is one with a period of 12.4 hours. There is a pattern of fluctuations characteristic of each day in a semi-lunar period.7. The reproducibility of the daily pattern in successive semi-lunar periods, and in successive years, is demonstrated.8. In general a strongly positive correlation is found between simultaneous hourly values for different groups of animals during the first seven days after collection. There is a general decrease in the extent of these correlations with time in the laboratory.9. There is strong evidence for a time-dependent variable affecting the size of the coefficients of correlation for simultaneous hourly values obtained during a wide range of times in the laboratory.10. Hourly values for single days of a semi-lunar period were correlated with the hourly values for comparable days of successive semi-lunar periods. The coefficients of correlation were positive and significantly different from zero for thirteen of the fifteen days of 1957, the exceptions being the eighth day after new or full moon and the day of new or full moon for which the coefficients were not significantly different from zero. For 1956 the exceptions were the day before new or full moon, and the second and eighth days after; the coefficients for these three days were not significantly different from zero. For the other twelve days the correlations were strongly positive.11. The relevance of these findings to an understanding of the phenomenon of biological rhythmicity is discussed.

1. The snail is able to differentiate between two weak magnetic fields, one oriented at right angles to the other.2. The relative influence of experimental north-south and east-west oriented magnetic fields in producing dispersion of snail pathways, or effecting a magnetoklinokinesis, displays both solar- and lunar-day rhythms.3. The relative influences of experimental N-S and E-W fields in effecting a predominantly counterclockwise snail-turning exhibits solor daily, lunar daily, and semi-monthly rhythms.4. The solar and lunar clock-regulated discriminatory responses for magnetic fields indicate the snail to be significantly oriented as if by internal magnetic compass needles which in turn are hands of horizontal solar- and lunar-day "clocks."5. Further arguments are advanced for concluding that organismic orientation to the earth's natural weak magnetic field is a normal organismic phenomenon.6. Implications of this demonstrated solar and lunar "clock-compass" capacity for the well-known sun-compass" and "moon-compass" orientations of animals are discussed. Also, the insurance value of a "clock-compass" capacity as a potential navigational system for animals when deprived of celestial references, is pointed out.

1. The direction, and mean amount, of turning in snails initially directed southward into a constant symmetrical, illuminated field displays a lunar-day rhythm with minimum turning about the time of moon-rise, and maximum turning at lunar nadir. There is also a lunar-day cycle of standard deviation of snail pathways, with a minimum about moon-rise and a maximum near moon-set.2. The response of snails to an experimentally augmented magnetic field also exhibits a lunar-day rhythm with maximum turning to the left at lunar nadir.3. The specific character of the lunar-day rhythm of the response to the experimental magnetic fields gives further support for the view that magnetic field is normally involved in snail orientation.4. The mean daily response of snails to experimental magnetic fields, expressed as differences from the response of controls in the earth's natural field, displays a semi-monthly rhythm. Maximum right-turning in response to a magnetic increase of 10-fold over that of the earth occurs one to two days before new and full moon, and maximum left turning just before the times of the first and third quarters of the moon.5. There is a synodic monthly fluctuation in mean daily standard deviations of snail paths with maximum deviation about two days before new moon and minimum deviation about the time of full moon.6. Some suggestive correlations are demonstrated between barometric pressure and the spatial orientation of snails in an environment constant with respect to all generally accepted orienting factors.7. It is pointed out that similarities in influence of some unidentified barometric pressure correlates on (a) magnetic orientation of snails, (b) general cellular oxidations, and (c) spontaneous activity cycles present reasons for postulating that the latter two phenomena are in some manner related to the magnetic field response and suggest that response to magnetic field is a widely distributed biological phenomenon.8. Evidence is presented that suggests there is a biological influence of a universal-time-related, rhythmic, environmental factor.

1. Records of the O2-consumption of Uca pugnaxwere obtained during the summer, autumn, and winter months and analyzed for diurnal and lunar-day rhythms.2. An apparent annual rhythm in mean rate of respiration is reported.3. The respiratory rate during December, January, and February was found to be approximately 50% of that during June, July, and August. During September October, and November, a gradual decrease in rate was observed.4. The form of the diurnal rhythm of respiratory rate showed no changes associated with time of year. Similarly, no seasonal changes in phase relations were observed.5. The range of both diurnal and lunar-day rhythms showed fluctuations but these were not clearly associated with time of year.6. The lunar-day rhythm was found to show changes as follows : June through September the curve was bimodal with maxima at lunar zenith and nadir; beginning in October and continuing through January the curve was unimodal with a maximum near lunar zenith; in February the curve was bimodal but with minima at lunar zenith and nadir.7. The unimodal curve of the transitional period is interpreted as being the resultant curve of a mixed population.8. The possible relationship of exogenous factors to these seasonal variations is discussed.

1. Mud-snails in a uniform field of all ordinarily controlled directional factors distinguish among geographic directions.2. Geomagnetism is involved in this directional sense. This was shown by rotation of a 5-gauss horizontal magnetic field which produced an orientational behavior correlated with that observed when the snails were rotated in the opposite direction in the earth's field.3. The response pattern to compass directions of the 5-gauss horizontal field was essentially the mirror-image of that observed for the earth's 0.17-gauss field.4. The directional response of the snails in the geographic field, and the concurrent correlated response to a rotating experimental magnetic field, vary parallelly with time and with influence of other, still undefined factors.5. Rotation of a 5-gauss horizontal magnetic field through the series of four compass directions, differing by 45° from the four cardinal directions, may produce a pattern of compass-directional behavior either paralleling or mirror-imaging that observed concurrently for the four cardinal directions.6. The compass-directional pattern of response of the snails shows a monthly or a semi-monthly variation related to moon phase. There is reason to believe that this is related to the moon through some mediator other than the ocean tides or nocturnal illumination.7. The compass-directional response to a rotating 5-gauss horizontal magnetic field, shows a monthly variation or a semi-monthly one. Whether it is monthly or semi-monthly, and its amplitude, appear to vary parallelly with comparable cycles obtained during the same period in response to equivalent changes in orientation in the earth's 0.17-gauss field. This gives additional support to the conclusion that geomagnetism plays an important role in these orientational phenomena of the snails.

1. The maximum orientational response of the snail, Nassarius, to an abrupt experimental reversal of the horizontal vector of geomagnetism occurs when the reversed field deviates least in strength from the earth's horizontal 0.17-gauss one.2. The responsiveness to field-direction change drops off very substantially even before the field strength has decreased to 0.1 gauss or increased to 0.4 gauss. It is already minimal at 0.8 gauss.3. The receptor mechanism for compass orientation in response to geographic direction is, therefore, exquisitely adjusted to the strength of the horizontal vector of geomagnetism, even in contrast with the total strength of geomagnetism.4. There is a persistent effect of experimental magnetic fields deviating in strength from the natural one which remains for at least three to five minutes following removal of the experimental field.5. Following exposure to reversed horizontal fields stronger than the earth's, southbound snails respond in the earth's field by clockwise turning, and following exposure to reversed fields weaker than the earth's, by counterclockwise turning, relative to their path in the earth's field following exposure to an experimentally reversed horizontal field of the strength of the earth's natural one.6. These results suggest that the receptor mechanism of the organism for very weak magnetic fields is highly specialized and adaptive, probably playing important roles yet to be disclosed.

1. Locomotor activity of the fiddler crab, Uca pugnax, was recorded in constant illumination following a variety of light regimes and in natural illumination.2. When pre-treatment caused a shift in the phase relations of the diurnal rhythm, a shift of activity peaks with respect to time of low tide was also observed; the shift in phase of the tidal rhythm was about the same number of hours as that of the diurnal rhythm.3. Under recording conditions of constant illumination the rate of progression of activity peaks showed systematic variations such that periods calculated for a few days would range from 24.0 hours to 25.8 hours. The apparent period was a function of the phase of the diurnal rhythm in which the peak occurred.4. When recording was carried out under conditions of natural illumination, the tidal rhythm showed a consistent 24.8-hour period.5. The imposition of a 24.75-hour cycle by four-hour supplemental light periods yielded an apparent period of about 25.5 hours for 11 successive days. This is interpreted as indicating successive daily phase shifts of both diurnal and tidal rhythms.6. The imposition of 24.75-hour cycles by means of two-hour supplemental light periods failed to produce any consistent effects on rate of peak progression.

1. The responses of the black chromatophores of Uca pugilatoras observed in legs autotomized and maintained in sea water are described.2. The pigment in legs from normal animals in the day (dispersed) phase becomes concentrated after isolation; that from normal animals in the night (concentrated) phase remains concentrated.3. The pigment in legs isolated from eyestalkless animals disperses in the daytime and fails to disperse when the legs are removed from 8 P.M. to 2 A.M.4. The activity on eyestalkless animals of a series of concentrations of eyestalk extract was determined in the daytime and at night. Four of the six concentrations tested were found to be more effective in the daytime than at night.5. The results clearly demonstrate the existence of a diurnal rhythm in eyestalkless animals and that the structures of the eyestalk are not necessary for this rhythm.6. The data provide strong evidence that a black-pigment-concentrating substance participates in the regulation of the chromatophore system of these animals.

1. Persistent rhythms of primary solar and primary lunar frequencies are described for O2-consumption of two species of fiddler crabs, Uca pugnaxand Uca pugilator, and for the salamander, Triturus viridescens.2. These rhythms appear to be statistical rather than overt ones. The forms of the rhythms become apparent only upon the averaging of several days of data.3. The form of the daily variation in O2-consumption shows a monthly variation, the form of the rhythm for a two-week period straddling a new moon being different from the form for a two-week period straddling a full moon.4. The hourly values in the rate of respiration show a significant correlation with the concurrent rate of barometric pressure change. In all three species, the rate of O2-consumption increases in a direct relationship with the concurrent rate of barometric pressure fall and decreases in a direct relationship with the rate of pressure rise.5. There appears, in some instances, to be a correlation with the absolute concurrent barometric pressure, but this is distinctly less. The correlation was a positive one for the two species of fiddler crabs and a negative one for the salamander.6. It was demonstrated that O2-consumption in all three species of animals displayed, therefore, imposed daily rhythms and in all probability, also imposed lunar rhythms.7. The striking similarity of the forms and phase relations of these imposed daily and lunar rhythms of O2-consumption to ones which are being described else-where for three species of plants, the alga, Fucus, and the carrot and potato, is pointed out. The rhythms are also compared with very similar statistical solar and lunar rhythms of opening and closing of the shells recently described for the oyster.8. There is a brief discussion of the environmental forces which might be involved, and speculation as to possible relationships between these imposed rhythms and the demonstrated endogenous rhythms in producing the temperature-independent overt rhythms observed in many processes in animals.