279 



61 in winter conditions of 1899 (January-March), and falling as low 

 as 1 to 17 in 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 nauplii, 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 PI. X. and XII., Pt. I.) jand 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. in 

 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 Cydopidcz may be inferred from an examination of the data 

 of Table I. An analysis of the seasonal distribution of the total 

 young and adult Cydopidce 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 Cydopidcs. 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 

 w r hen eggs are most common, and that this bears no constant relation 



