208 
anew the cycle of growth and reproduction whenever the favorable 
conditions prevail. 
There is in this species no hard lorica whose variable processes 
might serve to demonstrate to every observer its capacity for varia- 
tion. This is doubtless one of the reasons why we do not find a host 
of new species and varieties of Polyarthra as in the case of Brachionus. 
It is subject to considerable variation in size,and the swimming 
lamellz vary in length, width, and serrations. Hempel (’99) records 
Wierzejski’s var. euryptera in our plankton, and I have often 
observed it, but no record was kept of it since the characters which 
define it are not readily seen in plankton enumeration. Weber (’98) 
has mentioned, without designating by name, a long-spined variety 
which I find very common among the individuals which occur in 
the Illinois. 
This planktont is subject to attacks of internal parasites (S poro- 
zoa?) which infest it at the times of its maximum pulses, though 
never to the extent observed in the case of Bimerium in Brachionus. 
It is very frequently loaded down by Colacium, and some of the 
smaller peritrichous Ciliata are often found upon it. The absence 
of a hard lorica has served to obscure somewhat its food relations 
to whatever animals prey upon it. 
Polyarthra platyptera is a cosmopolite, and is apparently found 
generally in the fresh-water plankton. Jennings (’00) reports it as 
abundant in the waters of the Great Lakes, and it has been found 
generally in American waters. Zacharias (’98) and Marsson (’00) 
find it in pond and stream waters of Germany; Stenroos (’98) 
reports it asa predominant rotifer in the planktonand littoral regions 
of Finland waters; and Borge (’00) finds it in Swedish plankton. It 
has also been found to be an important constituent in the plankton 
of European streams. Skorikow (’96) finds that it is the most 
abundant rotifer in the summer plankton of the River Udy, consti-. 
tuting almost a third of the total rotifers. There are indications 
in his records of recurrent pulses, and the largest numbers are found 
in September. Zimmer (’99) finds it perennial in the Oder, but 
never abundant. Schorler (’00) finds it in the Elbe from April to 
September, with maximum in August. Lauterborn (’98a) lists this 
species among the perennial rotifers,and states that it is dicyclic in 
the Rhine and its adjacent waters, which he has examined quite 
thoroughly. The vernal sexual period begins with the appearance 
Prints ai, 
