261 



The following forms are of numerical importance in the order 

 named : C. bicuspidatus , young Diaptomus, Cyclops edax, Diaptomus 

 sicilbides, D. pallidus, Canthocamptus spp., and Cyclops albidus. 

 Cyclops prasinus, C. modestus, C. phaleratus, and C. serrulatus are 

 also found, but in such small numbers as to be of no quantita- 

 tive consequence. 



DISCUSSION OF SPECIES OF COPEPODA. 



Argulus sp. A small and apparently young argulid was found 

 in the plankton on August 10, 1897. Members of this genus are 

 abundant upon Amia calva and both species of Lepisosteus, all very 

 common fish in channel waters. 



Canthocamptus spp., including C. illinoisensis Forbes. Average 

 number, 78. Canthocamptus was found in the plankton in every 

 month of the year but June. The percentage of collections contain- 

 ing Canthocamptus is greatest (44 to 63 per cent.) in March-May and 

 November, and the numbers per m. 3 are highest in March-May, 

 when females, females with eggs, and nauplii all occur in their 

 maximum numbers. All records of totals in excess of 400 fall in 

 this vernal period with the single exception of one collection in 

 August, 1897. The largest number, 3,058 per m. 3 , was found 

 April 29, 1896. 



Canthocamptus occurs throughout the whole seasonal range in 

 temperatures, with smallest numbers and least regularity during 

 maximum summer heat in June- August. It is thus a planktont 

 of the colder rather than the warmer part of the year. 



The relations which hydrographic conditions bear to the occur- 

 rence of Canthocamptus in the plankton may be inferred from the 

 fact that of the 48 records in 1894-1899, 24 were made in rising 

 flood waters, 14 in falling flood stages within a few days after the 

 culmination of the rise, and but 10 in stable conditions or in declining 

 levels when flood waters of recent origin did not fill the channel. 

 From these facts it seems probable that Canthocamptus is in the 

 main adventitious in the plankton from its normal habitat in the 

 slime at the bottom and margins of the river and its backwaters. 



Over 88 per cent, of the total Canthocamptus recorded in the 

 plankton consists of nauplii. It may be that as is the case with the 

 young Ostracoda they enter the area of the plankton more readily 

 than the adults. Adults were found in the plankton only in 



