32 ILUNOIS BIOLOGICAL MONOGRAPHS [226 



on the posterior ventral portion is somewhat extended, making a bulge or 

 knob-like enlargement at that point. The dorsal anterior part is only slightly 

 inflated and slopes up to the anterior proboscis or lappet-like front end. This 

 is a triangular flat portion extending back behind the ciUated pits. The great- 

 est width of this triangle is nearly equal to that of the body in general but the 

 thickness is not more than one-haK the body depth, so that the mouth enlarge- 

 ment is accentuated. The shape of Turbellaria generally depends on the 

 amount of contraction or expansion but here the rather unwieldy body is 

 never much altered. That is, the animal is flexible and capable of contracting 

 within very narrow limits. The most contractile portion is, of course, just 

 behind the mouth region where the extreme flexibility of the pharynx demands 

 heavy muscular action, and the muscles of this part are much more thoroly 

 developed than those of any other species. Such a condition can be correlated 

 with the heavier kind of food. The regions of greatest contraction show 

 clearly in the preserved material, where the anterior tip and pharyngeal regions 

 are drawn back into the body integimient. 



The color appears white. Since the body is heavier and thicker than most 

 forms, it is opaque and not at all transparent. The heavier integiunent and 

 thicker layer of parenchyma obscure the intestinal contents and also the 

 intestine itself, so that there is no chance for any color to shine thru. Against 

 the dark backgroimd of water, these individuals stand out strikingly. 



The integimient is, of course, a one-celled layer, a Httle heavier in propor- 

 tion than that of other species. It is not so transparent as might be expected, 

 due to the rather solid outside walls of the cells, but in most respects is not very 

 different from the general t\'pe. The thickness varies with contraction. In 

 life, it is about 10/i, in mounted sections it is close to 17^ over all the body 

 except the anterior and posterior ends, where it is from 20/i to 25^. That the 

 cohesion of the cells is sUght is evident in prepared material where many of 

 the cells are practically pushed out or even completely dislodged from their 

 original position in the epithehum. WTien the whole structure is thus crowded, 

 the cells dove-tail into each other to some extent. They are then high colum- 

 nar, but more or less irregular, almost trapezoidal often, so as to fit compactly. 

 The portion containing the nucleus is the larger and with few exceptions is the 

 inner part. The nuclei are large and stain heavily so that they form the most 

 conspicuous part of the integiunent. In sections, they stand out against the 

 very fine delicate cytoplasm. The cilia are ver}'^ fine and long, from 16ju to 20/i. 

 They are of about the same size and are distributed evenly over all the body. 

 They move in waves from anterior to posterior as a general rule. The cilia 

 lining the sensory pits and mouth indentation are longer than the others and 

 very even. The ventral side is not so much differentiated as in most species 

 and the ciUa are little different from those on the dorsal side. At one point 

 near the posterior end, however, where the animal habitually attaches itself, 

 the epithelial cells are heavier and the cilia show a tendency to be short and 

 large. Another detail not so conspicuous is the presence of very small, clear 



