348 



The reasons for the iiiucli greater abundance of these worms in 

 the summer of 1922 than in the summer of 1920, when the average 

 oxygen supply was somewhat less at the lowest than in 1922 at upper 

 lake stations, are not clear unless we assume that the increase followed, 

 as it might quite naturally, the bringing in with the very unusual flood 

 of the winter and spring of 1922 of a new load of rich sediments, along 

 with a richer bottom bacterial flora, than were present in the summer 

 of 1920, following a more quiescent spring season. In fact, these 

 worms do not seem to be bound to their habitat so much by a quali- 

 tative as by a quantitative food relation, deriving their sustenance 

 as they do, without apparent selection, from the bacteria and other 

 living and dead organic matter in the soft bottom-sediments, which 

 they first swallow oi masse. So far as a low oxygen supply is con- 

 cerned, they seem merely able to tolerate it by virtue of their ability to 

 store haemoglobin and to effect a mechanical aeration of their imme- 

 diate surroundings, rather than to be in any true sense partial or obli- 

 gate to it. 



LEECHES 



Two species of leeches were taken in the upper lake in the summer 

 of 1922 : the first, Erpobdclla f<unctata (Leidy), in the channel opposite 

 Chillicothe; the second, Hclobdclla nephcloid'\i (Graf), in the wide 

 waters 3,600 feet eastward of mid-channel in the cross-section a mile 

 and a half above Spring Bay. The first species mentioned was taken 

 also in 1922 in the river at the P. P. U. Railway Bridge opposite the 

 lower part of the city of Peoria whei'e the pretty well aerated waters 

 of the lower lake are being mixed with the discharge from the main 

 sewers. ErpobdcUa punctata was noted by Baker (1922) as common 

 at stations in the Big Vermilion River in 1918-1920 where gas bubbling 

 was continuous in warm weather and where the mussels were dead ; 

 and by Weston and Turner (191T) as common at the upper and worst 

 polluted stations in the Covveeset below Brockton, Massachusetts. 

 Muttkovvski (1918), on the other hand, reported ErpobdcUa f'linctata 

 to be common along clean gravelly shores of Lake Mendota. 



Hclobdclla ncphcloldca, in addition to its occurrence in the upper 

 lake above Spring Bay at stations where distinctly bad X)dors were re- 

 corded, was also taken in the wide waters of the middle lake opposite 

 Mossville ; in the steamboat channel and the wide waters of the lower 

 lake opposite Fulton Street, Peoria; and. in company with ErpobdcUa 



