249] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 55 



This is due, not to thickness of material, but to compactness and a membrane- 

 like toughness. The inner line of the epidermis is also weU defined, tho not 

 as dark and sharp as the outer border. It is reinforced by a very thin basement 

 membrane which helps to make a firm base of attachment for the numerous 

 strands of muscle cells. Since almost the whole of the muscular system is, 

 to a greater or less extent, connected with the integument, this layer must be 

 able to withstand the stress of very nearly every contraction. The middle 

 part, between these two clear boundary walls, is very transparent, since the 

 cross walls show but little, and for this reason the shape and size of the epitheUal 

 cells is difficult to ascertain. This fact offers some explanation for the ease 

 and frequency with which the cells break apart. Most of the minor injuries 

 to the worm are tearings or breakings of this layer, which seem as easily healed 

 again. It is probably of great advantage that the cells, because of their very 

 slight connection with each other, are able to repair an injury very quickly. 

 Under conditions of injury when the tension is released either entirely or only 

 on one side, the cells very quickly become spheres, showing conspicuously 

 their primitive flexibility and lack of specialized form. This flexibility is 

 imperative when there is taken into consideration the demands made upon the 

 epithelium in the way of sudden contraction and expansion. There is an 

 incessant and constant change in the amount of strain brought to bear upon a 

 single cell or group of cells. For this reason, either in a small portion or in 

 the whole layer the shape and thickness do not remain the same for any length 

 of time and except when perfectly quiet the thickness is not the same for 

 any two parts of the body. Under some pressure the high columnar cells 

 become flat, almost scale-like, and the layers consequently very thin. In gen- 

 eral the average thickness ranges from forty to one-hundred micra. 



The external surface of the entire body is covered with a compact coat of 

 cilia. These are distributed very evenly, not being grouped. There is also 

 little variation in size as they are not longer or larger even at the tips 

 of the head and tail. No sensory pits with large cilia are present and those 

 cilia around the edge of the mouth are also of the same general dimension. 

 In shape they are slender spines with the base sUghtly larger than the upper 

 portion, which is drawn out to a long, fine point. The length averages near 

 the epithelium thickness, with a diameter of three to six micra, and they appear 

 as extremely fine transparent lines. The vibrations are rapid and altho gen- 

 erally the waves of motion are from anterior to posterior, yet upon occasion 

 they may be reversed. Then in different portions the action may be at 

 times entirely independent of the surrounding surface, continuing in some 

 portions long after the main part of the animal has been killed. 



Other structures included, or at least connected, with the integument are 

 the rhabdites, which are arranged in two tracts in the anterior portion of the 

 head. These tracts are symmetrical and are placed one on either side of the 

 middle fine in the dorsal part of the parenchyma. The anterior half of each 

 group is a broad, fan-shaped portion which extends to the very tip of the head 



