370 



Birge — The Crustacea of the Plankton. 



were found in the same species during the winter of 1895-6 and 

 indeed similar tables could be constructed for any species fairly- 

 numerous, and neither increasing nor declining in numbers. 



On July 21 and August 15, 1896, a series of catches was made 

 extending across the lake some 5 kilometers, at approximately 

 equal distances. The result of the latter catch is given in the 

 accompanying table; the other was substantially the same. 



Table XXVI. — Collections on August 18, 1896, expressed in thousands 



per square meter. 



Diaptomus 



Cyclops 



D. pulicaria 



D. hyalina 



D. retrocurva 



Diaphanosoma 



Chydorus 



Leptodora 



Ergasilus 



Nauplii 



Corethra 



Asplancha 



Total Crustacea 



I. 



II. 



III. 



27 



51 



40 



184 



203 



142 



57 



31 



3.3 



37 



31 



15 



13 



16 



7 



10 



18 



13 



35 



217 



184 



0.7 

 17 



1.5 

 16 





3.3 



241 



337 



? 



6 



8 



1.1 



114 



101 



33 



631.7 



921.5 



*407.6 



IV. 



80 

 136 



33 

 11 



27 

 154 



0.2 

 8.9 

 236 

 1.2 

 66 



VI. 



74 



127 



33 

 3.3 

 33 

 174 

 0.5 



686.1 



134 

 1.3 



40 



678.8 



VII. 



83 

 145 



38 

 20 

 49 

 147 

 0.5 

 0.5 

 167 

 4.4 

 45 



650 



* No nauplii included. 



The number of Cyclops when at its maximum showed sur- 

 prisingly little variation. In 1895 from May 1st to June 6th, 

 26 catches were made on 13 days. The catch ranged from 

 10,000 to 20,000 individuals actually caught. In 1896, 18 catches 

 were made on 16 days. The numbers ranged from 9,000 to 37,000. 

 A figure is added (Fig. 21) showing the number of Cyclops 

 caught during the year 1895. It will be seen that the diagram 

 gives no warranty to the conclusion that this species appears 

 in swarms. Similar illustrations could be taken from any year, 

 and from almost any species, with the qualification that the range 

 in number is greater in the case of those species whose num- 

 bers are small. 



