269] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 75 



the rhabdite tracts very conspicuous. It was flat and moved constantly in all 

 directions. The tail was not pointed sharply but tapered to a short, blunt 

 tip. Some of the specimens were much inflated with young worms, but this 

 inflation was almost entirely effected on the upper surface alone. At a point 

 just posterior to the eyes the body wall very sharply rounded upward, making 

 the head only a flat projection on the ventral anterior border of the whole mass. 

 The posterior end narrowed more gradually. In any collection a good many 

 young individuals appeared. These were always more regular in outline, the 

 head and tail ends tapering not at all and the width being nearly the same 

 thruout the body. 



Another rather distinctive detail was conspicuous in specimens not swollen 

 with young. This was a definite fold or angle about midway between dorsal 

 and ventral surfaces and running from near the anterior end far back, almost 

 to the tail. This fold is marked by tliickness of the epithehal cells and by two 

 stripes, one dark, one hght. It somewhat resembles the middorsal ridge of 

 Mesostoma tetragoniim. However, in this latter species the form is exceedingly 

 flat and broad, causing the ridge to appear as a mere projecting fold on the upper 

 body surface. In my specimens, on the other hand, the cyUndrical shape 

 carries the ridges outward making them sharply angled corners, and giving a 

 very characteristic appearance to the body as a whole. In individuals dis- 

 tended with young worms the folds are entirely obHterated. The line of 

 demarcation between the dorsal and ventral surfaces is also definitely developed 

 as a hghter, heavier streak. Since the ventral surface is flat, this line is never 

 to be seen unless the animal lies partially turned on one side. This ventral 

 line and the lateral fold are very much alike in appearance and structure. 



The color varies from a dehcate to a dark brown and is lodged in the par- 

 enchyma. For this reason the anterior head region in which there is little 

 room for middle cells is entirely clear. The color is not the same over all other 

 parts of the body but is concentrated in a wide dark strip down the middle of 

 the dorsal surface. Then, also, the folds and angles along the edge are banked 

 by a heavier band. Young individuals show very pale yellow and those just 

 born are entirely without color. 



The rhabdites occur in the parenchyma in close relationship with the color 

 cells and are sharply defined and clear. They He in groups of six or ten or are 

 promiscuously scattered. The head also shows heavy tracts. In shape they 

 are straight rods with rounded ends and without any variation in diameter. 

 Under a lens the rhabdites give the parenchyma a checkered, rough appear- 

 ance. Two details of digestive tract structure which stand out clearly are, 

 one, the narrowness of the intestine, the other the shape of the pharynx. The 

 intestine instead of spreading out through the parenchyma, is rodlike and 

 consequently more compact and of greater depth than that of most species. It 

 extends only as far forward as the eyes, a possible reason being the lack of 

 space in the head. The pharynx rosette is here almost spherical as differing 

 from the flat fuimel found in other related forms. The iimer dorsal surface 



