16 ILLINOIS BIOLOGICAL MONOGRAPHS [114 



Oligochaeta. These aquatic earthworms were abundant in places 

 along the shore of Salt Fork, Sparganophilus eiseni Smith occurred in 

 abundance in the mud on the margin of Salt Fork at Homer Park, and 

 immature worm of this genus, as well as cocoons, were collected from near 

 the bridge below St. Joseph to the bridge above Sidney. A single specimen 

 of Helodrilus chloroticus (Savigny) was collected in the Salt Fork near St. 

 Joseph. This species has not heretofore been recorded from Illinois and 

 its occurrence at this locality adds another species of earthworm to the 

 State list (see Smith, 1915:557). Limnodrilus was abundant in bottom 

 sludges from the Salt Fork, from Urbana to near St. Joseph. Tubifex was 

 also found, but not as abundantly. 



Hirudinea. Two species of leeches occurred in several places in Salt 

 Fork, below St. Joseph. They were nowhere abundant. The two species 

 are: Erpobdella punctata (Leidy) Moore and Placobdella rugosa (Verrill) 

 Moore. 



CRUSTACEA 



Three species of crayfish (Cambarus) were determined by Professor 

 Smith from the material collected in Salt Fork and other parts of the Big 

 Vermilion River. The most abundant was Cambarus propinquus Girard, 

 which was found commonly from bench mark 655 to the Big Vermilion 

 River below Middle Fork. It is especially abundant in the latter place 

 where an individual may be found under every piece of stone. The same 

 species was abundant in Stony Creek and in the Salt Fork near Muncie. 



Cambarus blandingi acutus Girard, both young and adult, occurred in 

 the upper parts of Salt Fork, but preferred a mud bottom rather than a 

 rock bottom. Cambarus immunis Hagen was collected only at bench mark 

 655 where it was rare. 



It will be noted that Cambarus is first met with about two miles below 

 St. Joseph and twelve miles from Urbana. The only ones seen here were 

 dead. Living crayfish begin to appear in abundance about six miles below 

 St. Joseph or sixteen miles below Urbana. Crayfish and mussels both be- 

 come common or abundant at about the same time, viz., fourteen and 

 sixteen miles below the source of sewage pollution at Urbana. This agree- 

 ment in distribution indicates the close relationship between these other- 

 wise diverse groups of animals, as regards resistence to septic conditions, 

 neither being able to thrive under pollutional conditions of the bottom. 



AQUATIC INSECTS 



Aquatic insects are doubtless abundant during spring and early summer 

 in the lower parts of Salt Fork and in the Big Vermilion River. A few 

 species were collected incidentally at some of the molluscan stations. 

 These are listed in Table I, in the order of their occurrence in the stream. 



