218 



PULSES OF TRIARTHRA LONGISETA. 



This is an exceedingly variable species. It varies in the relative 

 length of the three long setae, in their spinosity, and in the location 

 of the posterior one. Many of the individuals in our waters resemble 

 the form described by Plate ( '85) as T. terminates. The long-spined 

 form described by Zacharias ( '94) as var. limnetica is also abundant. 

 It is doubtful if either form is worthy even of varietal distinction. 



This species has been reported only from Lake Erie and the 

 Illinois River in this country, and seems to be rare in the former. 

 Weber ('98) finds it abundant in the plankton of Lake Leman; 

 Burckhardt ('00 and 'OOa) reports it as wide-spread and almost 

 perennial in Swiss lakes, but with its maximum in December- 

 February, and slight development during warmer months. Borge 

 ( '00) finds it to be one of the common rotifers in the summer plank- 

 ton in Sweden ; Marsson ( '00) reports its perennial seasonal range in 

 several German waters, with greater numbers during the warmer 

 season. Apstein ('96) gives it a perennial distribution in Lake 

 Plon, with larger numbers in June-November, and maximum in 

 June- July or August. According to Seligo ( '00) the species is per- 



