217 



terborn ('98a) lists it among the summer rotifers of the plankton 

 of the Rhine. The genus is in need of a thorough revision in the 

 light of possible variation.* 



Taphrocampa annulosa Gosse. Average number, 71. Found 

 in September, at 73. Evidently adventitious. 



Triarthra longiseta Ehrbg. Average number of individuals, 

 3,147; of eggs, 293. This species was about twice as abundant in 

 the stable conditions of 1897, and was present in less than half these 

 numbers in the recurrent floods of 1896. 



It is a perennial species, having occurred in every month of the 

 year. The continuous occurrences and the larger numbers lie in 

 all years between May and October and above 60. In 1898, only 

 about 3 per cent, of the total individuals were found below this 

 temperature. With the exception of the vernal pulse of 1898 all 

 of the larger numbers were found in the period of maximum heat. 

 The optimum conditions for this species are thus found within that 

 period and above 70. 



The seasonal routine of the species is varied somewhat from year 

 to year. There is usually a slight vernal pulse larger than usual 

 in 1898 and this is followed by recurrent pulses throughout the 

 summer. The season closes without a predominant autumnal pulse, 

 and after September the numbers fall and the occurrences become 

 sporadic until the following April. 



The pulses of this species are listed in the following table, which 

 gives their locations and temperatures. 



Of the 21 pulses recorded, 18 are within the periods of the plant 

 pulses shown in Plates I. and II. Of these 18 there are 8 which 

 coincide with these plant pulses, 9 which follow after a short interval, 

 and 1 which shows no such relation. The dependence of the pulses 

 of Triarthra upon food conditions is suggested. The pulses of 

 Triarthra will be found on examination of Table I. to coincide in 

 1898 in the main with those of the total Plointa. 



The pulses are never very large, and the evidences of reproduc- 

 tion are not well defined. Attached summer eggs attend the larger 

 pulses, and free winter eggs of the species were found in October- 

 November in 1898. In previous years free or attached eggs attended 

 vernal or summer pulses at times. The evidence indicates a poly- 

 cyclic habit. 



* See Rousselet, '02. 



