273] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 79 



forms could be easily seen gliding around in the more open places. With the 

 aid of a small magnifying glass much of their movement could be watched. 



In general the body motions are very regular, much more so than in types like 

 Mesostoma, which crawl about in the depths of algal labyrinths. As von 

 Graff says, Gyratrix is extremely contractile but this power is manifest only 

 in a longitudinal direction. That is, the head and tail regions may be so 

 drawn together that the shape is nearly that of a sphere; but, on the other hand, 

 the head is rarely twisted or turned to one side, and the animal never bends 

 upon itself, but always maintains a straight position. It glides evenly and 

 often rather rapidly in nearly a direct course never pushing, here and there, 

 or hunting in one place and then another. The locomotion is more by means 

 of cilia than by muscular contraction of the body wall. The cilia are very 

 small and practically the same size. The food seems to be gotten, 'hit or 

 miss,' rather than hunted out. The individuals sail aimlessly about till they 

 happen upon food. When coming in contact with something hard or un- 

 favorable the animal quickly contracts and then changes its course — never 

 investigating or crawling about as some species do. 



The length of European species is given as 2mm. Few of my specimens 

 measured over 1mm. This however, may have been due to conditions of 

 that special locality, or to the season of the year. The body is slender, the 

 greatest width being only one-fourth the length, and slightly flattened on the 

 ventral surface. The shape is somewhat significant. The widest region is far 

 posterior, generally at a point between the two sexual pores. The posterior 

 end is bluntly rounded while the anterior part narrows evenly and gradually 

 to the proboscis tip. There is no pigment so that by transmitted hght the 

 body is very clear, even the thickest part being almost transparent. To the 

 naked eye the color, as seen against a dark background, is a thin greenish 

 white. This is noticeable since most forms are quite dense or opaque in such a 

 situation. 



This transparency is due probably to the lack of a large amount of paren- 

 chyma. The slender body is nearly filled with the few body organs and since 

 there is Uttle plumpness, there is little room for large interstitial cells. There 

 is also little evidence of many muscle fibres. The body wall cannot be drawn 

 in at any point as in other species. The few muscles needed are developed in 

 connection with the reproductive vesicles and the anterior body tip. Then 

 another thing tends to increase the transparency; there are no rhabdites, so 

 that the epithehum seems very thin. Also the intestinal part of the digestive 

 system has a deHcate wall, allowing whatever food material is within, to be 

 plainly seen. Most of the specimens however, showed only small amounts of 

 pale greenish material in the intestinal tract. They are evidently not heavy 

 eaters. The rosette of the pharynx thus shows quite plainly, a Httle anterior 

 to the middle. It is made up of rather slender thin-walled cells, so that the 

 divisions are not very easily made out. The outside boundary, also, is not 

 thick, since there are no extra cells. 



