2 
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 Plowma. 
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. 
