227] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 33 



rhabdites; where the integument is expanded they lie flat, parallel to the sur- 

 face but scattered and more or less irregularly placed with regard to each other. 

 Under heavy contraction, when the cells are narrow and deep, the rhabdites 

 are perpendicular to the surface, arranged evenly as a layer on the outermost 

 surface, just beneath the cilia. They are even, smooth little rods with blunt 

 ends and are all of the same size, and occurring over the whole surface of the 

 body. The influence of kiUing and fixing agents often slightly swells them to 

 transparent knob-like bodies. 



The integument is very closely related to the muscular system which Hes just 

 beneath it. As Ott (1892) states, the outside layer, next the epithelium, 

 is composed of circular strands while that inside toward the parenchyma is 

 longitudinal. The circular muscle cells are many and make a single row of 

 almost round cells extending the length of the body. This row of cells is 

 interrupted at the fission plane and in the region of the ciliated pits. The 

 several individual strands are often 20fj. apart, often side by side. The cyto- 

 plasm is granular and stains heavily. The longitudinal cells are very slender 

 strands with the nuclei showing as tiny enlargements along at different points. 

 They are not many in number and are scattered. Very few run directly longi- 

 tudinal, most being shghtly oblique or extending from one portion of the 

 epithehum to the intestine, or to some other part. The layers around the 

 phar}Tix and mouth have the cells lying much closer together and on the whole 

 they are longer. Around the wall of the intestine, the circular strands and also 

 the longitudinal are almost embedded among the digestive cells. They show 

 somewhat scattered and heavily stained between the outer ends of the light 

 large ceUs which make up the assimilative layer. When the intestine is filled 

 and the small amount of parenchyma pushed away, these muscle strands are 

 close to those of the body wall. The most striking characteristic of this system 

 is the extremely small number of strands or fibers thruout the whole structure. 

 There is much less true muscular contraction than in almost any other family 

 of this group and a great part of the movement of cells is due to changes in 

 physiological condition. 



The parenchyma is extremely vacuolate and the cells are very delicate. 

 Most of the support given the different organs and also the stiffness of the 

 body as a whole, is due to the turgidity of these few parenchyma cells and to 

 the watery protoplasm which fiUs the vacuoles. As Ott finds for Stenostoma 

 leucops, the space between the intestine and body wall contains very few cells 

 and the only material to be displaced under varying conditions is the body 

 fluid. The largest mass of parenchyma is that just posterior to the brain and 

 surrounding the anterior portion of the pharynx. In prepared sections, this 

 shows as a very irregular network with very few nuclei, many of these connect- 

 ing strands are broken and the cell bodies torn. The body of the cell is rather 

 small but varies somewhat. The nucleus is round, and stains deeply, showing 

 large granules. There are generally five or eight longer or shorter threads or 



