279 
61 in winter conditions of 1899 (January—March), and falling as low 
as 1 to 171n 1896. This was a year of recurrent floods, but its ratio 
is in sharp contrast with that of 1898 (1 to 30), also a year of con- 
siderable hydrographic disturbances during the summer. The adult 
population was reduced during this year, and especially during the 
summer floods, but the nauplii do not fall conspicuously below 
those of other years. It would therefore seem that the deleterious 
action of flood conditions operates more effectively upon the adult 
and young than upon the nauplii. This fact may be due to the 
relative absence of spines and hairs on the nauplu, structures which 
gather silt and load down the larger forms in the flood waters. 
The greater number of young and adults in 1896 as compared 
with 1898 may be due to the more gradual rise of the floods of the 
former year (see Pl. X. and XII., Pt. I.) land the proportionally 
greater amount of silt in the more sudden floods of the latter. 
The ratios given in the table are of course subject to the error 
arising from the uneven seasonal distribution of the collections in 
some years, and to that arising from varying location of the collec- 
tions on the pulses, especially on those of greatest amplitude. An ad- 
ditional error arises from the leakage of the smaller nauplii through 
the meshes of the silk net. I have found on experiment that they 
will thus escape under pressure of a column of water only 3-4 cm. 1n 
height. Their dimensions are such that the smaller individuals 
can pass through the meshes of even the No. 20 silk. It seems 
probable that ratios of nauplii to adults are actually greater than 
our records indicate. 
The relationship which the pulses of nauplii bear to those of the 
adult Cyclopide may be inferred from an examination of the data 
of Table I. An analysis of the seasonal distribution of the total 
young and adult Cyclopide and of the nauplii reveals the fact that 
in all seasons in which collections at approximately weekly intervals 
were made, their pulses coincide in a majority of cases in their 
maxima, and when the coincidences do not occur the maximum of 
the nauplius pulse appears in the collection of the week following 
that of the young and adult Cyclopide. This appears less constantly 
and clearly in the disturbed hydrographic conditions of 1898 (Table 
I.) than in the records of more stable years. 
Apstein (96) finds that nauplii of Copepoda are most abundant 
when eggs are most common, and that this bears no constant relation 
