36 ILLINOIS BIOLOGICAL MONOGRAPHS [230 



sources; that is, this species is entirely a scavenger in its habits. Altho swim- 

 ming free when disturbed, and often at other times quietly gliding around, it 

 pays no attention to any food material and only feeds when hunting thru the 

 thick bottom debris. This accounts for the fact that the intestine is always 

 very dark in color. But in general the Hfe-habits are very similar to those of 

 related forms. 



In a study of the anatomical details a few characters are significant. The 

 average size is about 1mm., altho, of course, there is the same variation as 

 in other members of the genus. The shape, however, is more characteristic 

 and definite. It is very slender with little difference in diameter. The relative 

 proportion of width and depth to length is about one to eight, which is quite 

 different from some others where it is one to five or six. The anterior tip in 

 front of the mouth is shorter than in most species, a condition which 

 gives the head a much shorter, blunter aspect. The shape of this end is also 

 less pointed than is often found. The posterior portion is short, that is, the 

 diameter of the body remains the same to a point about one-fifth the length 

 of the body from the end. Then the sloping off to the posterior tip is very 

 rapid and this end is nearly as rounded as the anterior end. The body, altho 

 so regular, is not at all rigid and bends easily. The color is light, except in the 

 intestinal region wliich generally shows up quite dark, making a- decided con- 

 trast between the anterior quarter where the head and pharyngeal portion is 

 nearly transparent, and the heavy body part. 



The integument is very thin, delicate, and transparent, which allows the 

 internal condition to be easily seen. The cilia are short and very evenly dis- 

 tributed thruout, being only slightly longer in the mouth region. The rhab- 

 dites correspond closely with those of other forms, being very small, regular 

 rods scattered thruout the epithelium and lying parallel to the surface. Several 

 are often grouped, lying side by side, but many are single. The parenchyma 

 is exceedingly transparent and is concentrated in the head region anterior to 

 the digestive cavity. The posterior triangle between the intestine and the tip 

 is very small and generally almost obliterated. The layer enveloping the 

 intestine is so thin as to be very nearly lacking, its presence being demonstrated 

 only at the fission planes where it forms a mass gradually increasing and pinch- 

 ing off the digestive cavity. 



The most noticeable structural character, is the digestive tract. The 

 mouth expands into a somewhat irregular round opening, but does not contract 

 to such a small cavity as in some species. The mouth indentation is a deep 

 fimnel rather heavily muscular which is not so flexible as in types having 

 the habit of swallowing large masses of food material at one time. This funnel 

 leads to the pharyngeal cavity which is long and narrow. The wall here is 

 very transparent and the outside cells small and almost invisible. The 

 connection with the intestinal portions is very sharply marked by a deep con- 

 striction. Except when the opening is partially expanded, the pharynx appears 

 pinched off com.pletely. The lumen of the digestive cavity is small and fairly 



