512 



Collections of the bottom gases were made again at Morris Sep- 

 tember 5 ; and with a view to learning how far down the stream the 

 bottom conditions found at this point were continued, like collections 

 were made at Henry September 6, and above the dams at Marseilles. 

 The chemical results are as follows. 



Sludge Worms. — It is clear that neither plants nor animals re- 

 quiring oxygen could live in these sediments at the bottom of the 

 stream. The abundance of Tubiflcidce {Limnodrilus and Tubifex) 

 imbedded in this sludge is explained by the extraordinary respiratory 

 capacity of these worms, and by the fact that their respiratory struc- 

 tures are situated at the anal end of the body, which projects above 

 the bottom and is kept continually waving back and forth. Their 

 circulatory system likewise specially adapts them to life in water with 

 a minimum amount of oxygen, the blood containing sufficient hemo- 

 globin in solution to give it a red color, and being kept in active 

 circulation through a closed system of blood-vessels. 



Composition of the Sludges. — The following is a table showing the 

 general composition of the sludges themselves according to analyses 

 kindly obtained for us by the division engineer of the Sanitary^ Dis- 

 trict of Chicago, Mr. Langdon Pearse, the samples for which were 

 furnished by ourselves at his request. 



The greater specific gravity, the diminished moisture, and the 

 smaller percentages of nitrogen and of fats in the sediments above 

 the Hem-}' dam, all indicate less organic matter in proportion to the 



