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 oiPolyarthra as in the case of Brachionus. 

 It is subject to considerable variation in size, and the swimming 

 lamellae 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 (Sporo- 

 zoaf) which infest it at the times of its maximum pulses, though 

 never to the extent observed in the case of Bimcerium 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 as a predominant rotifer in the plankton and 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 



