40 ILLINOIS BIOLOGICAL MONOGRAPHS [234 



Several specimens of another species have been found in the same pond with 

 Strongylostoma. Superficially, they appear like large Stenostoma, except 

 that they are a more opaque white and are never seen swimming free in the 

 water but crav/ling over the surface of the aquarium, hunching along, as it 

 were, one part of the body at a time. The movement is slov/ and uneven, with 

 great difiiculty, by means of muscular contractions, slight waves passing 

 over the body very slowly. When at rest, the animal lies crouched and some- 

 what drawn up with the head bent a little to one side and more or less of another 

 little bend at som^e other point. When in motion, the head is hardly ever held 

 straight •with the body, but is constantly moved this way and that, as tho 

 investigating the surrounding surface. 



In size the individuals vary from 0.75 to 2.1 mm. in length, the measure- 

 ments taken when the animal is as extended as possible. The width was hard 

 to ascertain because of its great amount of variation, never being the same for 

 any length of time. The average was 0.3 to 0.7 mm. for a point about the 

 middle of the body. The depth was nearly that of the width except in the tail 

 region, where there was some flattening. As a whole the animal is very n^rly 

 cylindrical. The head end is as broad as any part of the whole body, but 

 very amenable to change, so that often it appears pointed. In a lateral view 

 it shows a slope to the ventral surface, making a blunt point. At about the region 

 of the eyes, nearly one-fourth of the body distance back from the anterior tip, 

 there is a very slight constriction, which is gradual in slope and does not 

 appear unless the animal is quiet and fully extended. Posterior to this the 

 body diameters are very nearly the same as far back as the last one-fourth, 

 where there is a gradual and even narrowing toward the truncated tail. The 

 ventral surface is somewhat flat, but the lateral surfaces very gradually round 

 upward so that there is only a very narrow, entirely bottom surface. Color 

 seems to be lacking. The opacity apparent to the naked eye disappears under 

 the microscope, showing only a very transparent body. Even the outline is 

 not as definite and clear-cut as in many other white forms. The intestine 

 shows somewhat darker than the clearest portions but the outline is almost 

 invisible. The eggs alone are dark and thick, but have no especial color. The 

 atrium seminalis and sexual pore are very gray-orange wdth smootli waUs 

 distinguishable from the cellular parenchyma. This parenchyma gives a pale 

 greenish tinge to the body as a whole. 



The epithelium is thin and clear, the cell outlines hardly visible and even 

 the inner margin not distinct. It is of nearly even thickness over the body 

 except on the ventral surface of the tail where it is made heavier by cells 

 specialized as a holdfast. There is no such highly developed point on the head, 

 since the animal nearly always holds the anterior end a little elevated, prevent- 

 ing any contact vidth the substratum. The rhabdites are very inconspicuous, 

 being small, regular in shape, and clear. They are straight rods scattered 

 thruout the integument, to some little extent grouped together in threes or 

 fours but generally lying in various positions, except never end to end. The cilia 



