Ergasilus — Naup Hi. 333 



Ergasilus depressus Sars. 



This animal is about the same size as Cyclops, although 

 readily distinguishable both by color and form. I am not sure of 

 the correctness of the specific identification, although lean see no 

 differences between my specimens and Sars' description. It is 

 present at all seasons of the year, ordinarily in very small num- 

 bers. More than one individual is rarely found when one-tenth 

 to one-twentieth of the catch is counted. This number is so small 

 and the resulting probable error in computing averages so great 

 that it has not been thought profitable to state the numbers in 

 terms of a square meter of surface, and to include them in the 

 total number of limnetic Crustacea. 



Ergasilus is present throughout the year, although it may 

 often be missed for long periods from the collections. It was 

 first noticed in July, 1895, although doubtless present before, 

 .and from 1 to 9 specimens were seen in each collection. The 

 number increased during the latter part of August and in Sep- 

 tember, when from 10 to 13 specimens were found, indicating 

 nearly 10,000 per sq. m. In the latter part of September the 

 numbers rose to a maximum of 27-30 specimens, or nearly 27, 000 

 per sq. m. In October only 1 to 5 were present, and the species 

 was found occasionally during the winter and spring in single 

 specimens. In July and August, 1896, it became more plentiful; 

 about as is 1895. But no such large number was found in Sep- 

 tember as in the former year. The animal seems to prefer the 

 stratum of water just above the thermocline, but is not confined 

 to this layer. 



Copepod Larvae — Nauplii. 



The dredge with which my study was carried on until the 

 middle of July, 1896, was provided with a bucket whose open- 

 ings were closed by a wire mesh of 1-100 in. This, while re- 

 taining the Crustacea and a great part of the nauplii, did not 

 retain all of the latter, so that no study was given to these 

 larval forms until work began with the silk net. The following 

 table shows the average number of larvae from the middle of 

 July to the end of December, and also the numbers found in 



