88 University of California Publications in Zoology [Vol.22 



BdeUoida 



Station I Station II Station III Daily Hourly 



Number of forms recorded 6 6 3 3 2 



Individuals per cu. meter.. 349,312 47,105 56,806 120,831 225,414 



These large averages are perhaps unfair, inasmuch as they are 

 largely influenced by the assemblage of unidentified forms which was 

 assigned to the Bdelloida because it seemed almost certain that all but 

 a very few belonged to that group. 



The only genus recorded is Rotaria and it is itself a heavy con- 

 tributor to the plankton. Probably some other genera are included, 

 especially in the unidentified list. The larger numbers at Station I 

 indicate a distinct preference for sewage, a condition directly opposite 

 to that of the Khizota. The representation in February and March at 

 Station I is much heavier than at any other time of year, the maxi- 

 mum falling on March 12. This is very largely due to the abundance 

 of Rotaria rotatoria. The maxima at Stations II and III came in 

 October and August respectively, due to the greater influence of the 

 unidentified assemblage at those stations. It must be acknowledged 

 that the data for this group do not lead to satisfactory conclusions. 

 Some of the difficulty is due to the characteristics of the group and 

 some to the difficulty of identification in preserved material. 



Discussion of Species 

 Rotaria neptunia Ehrbg. 



Station I Station II Station III Daily Hourly 



Average 14,987 2,331 3,829 1,961 492 



Identification usually positive. Occurrence most constant in cooler 

 months of the year at all stations. Much more abundant at Station 

 I, thus showing preference for sewage. Not recorded in April, May 

 and June at Station I and but rarely in July and September. Num- 

 bers small in March, species well represented in other months. Maxi- 

 mum number recorded twice, October 15 and November 5 at tempera- 

 ture of 18.5° C, which is probably near the optimum. Small num- 

 bers in early January were probably due to the temperature being 

 below 10° C. Flood conditions may be responsible for absence in 

 April, May and June while high temperature accounts for it in July. 

 The reason for absence in September after being recorded in August 

 is not clear. 



While definite pulses were present, there is not much regularity of 

 appearance. 



