1923 ] Esterly: Marine Copepoda at La J oil a 427 



different, it was followed by decreased numbers for three weeks, and 

 in the fourth week after the actual maximum there appeared another 

 marked increase which corresponded to the maximum of 1918. A 

 comparison of the two columns in table 4 that deal with A. clausi 

 will show that there is a broad similarity in the occurrence of this 

 copepod in the corresponding weeks of the two successive years. 



Acartia tonsa reached its actual maximum in the third week of 

 July in both 1917 and 1918. For the next two weeks the numbers 



TABLE 5 



Corrected Values for Number op Animals per Haul at the Six Periods op 



Collecting. 



The results of the two methods of correction are combined. 



Number of animals per haul Mean 



S a.m. Noon 4 p.m. S p.m. Midnight 4 a.m. abund. 



First year: 



A. tonsa 65 22 72 150 312 261 147 



A. clausi 97 45 97 212 199 210 143 



Small calanids 140 94 81 233 99 96 141 



Podoplea 1304 990 1240 1425 1353 1480 1259 



Second year: 



A. tonsa 135 29 101 301 408 401 229 



A. clausi 85 38 40 276 363 391 282 



Small calanids 130 51 93 126 119 136 91 



Podoplea 1211 884 987 1512 1439 1913 1384 



Average for the two 

 years : 



A.tonsa 100 25 S6 275 360 331 183 



A. clausi 91 41 63 244 281 300 212 



Small calanids 185 72 87 180 110 166 216 



Podoplea 12.57 937 1113 1468 1396 1696 1321 



were relatively much reduced in both years. After that there was a 

 noticeable increase during one week, which was followed by a marked 

 drop (August 12-18). The close similarity between the two seasons 

 ends there ; but whatever the differences mean the points of corres- 

 pondence are worthy of note. 



In any such investigation as this, the matter of diurnal distribu- 

 tion is sure to come up. A casual examination of the collections will 

 show that there are marked differences in abundance between the day 

 and night hauls. In table 5 there is an arrangement of the corrected 

 figures, and in table 6 are indicated the probabilities that certain of the 

 differences in table 5 are significant. 



The changes in abundance that occur between 4 a.m. and 8 a.m. and 

 between 4 p.m. and 8 p.m. are the most important ones, since they 



