251] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 57 



lose their straight position and appear more or less warped. Altogether the 

 epithelium with its cilia and rhabdites is a very characteristic part of the body 

 structure. 



MUSCULAR SYSTEM 



The muscular structure forms not a layer but a finely ramifying system of 

 most delicate fibers radiating in all directions thru the body mass. The fibers 

 extend from one part to another, intertwining and interlacing into a regular 

 tangle which is complete enough in itself, if the rest of the body mass were 

 taken away, to preserve almost a perfect outline of the body organs. One has 

 only to watch the constant movement of many portions of the animal make-up 

 to realize the existence of very many contractile fibers. Since the worm is 

 very sensitive in every part of its structure, every part must be capable of quick 

 response. Since the habitat makes it necessary to find a way in any jungle 

 of water weed or debris, each individual must find it possible to wriggle thru 

 any maze within which food and shelter lie. 



The anterior end is, of course, the most sensitive region and, as a conse- 

 quence, its musculature is well-developed, connecting the several head lobes 

 with each other and with parts farther back. The muscle strands are extremely 

 delicate slender threads, most of them ventral to the brain ganglia. A very 

 few extend directly across from side to side, a few others are oblique, but by 

 far the most run from the different parts of the head back to the intestine or 

 to the integument of posterior parts. They might be described as clusters of 

 fibers starting from some point in the main body region and spreading out fan- 

 like for their insertion somewhere on the inner surface of the head integument. 

 Since the head is very flat and there are few dorso-ventral strands, the whole 

 complex forms a plate-hke layer, very thin in front and becoming thicker 

 toward the neck region. 



The muscles of the posterior half for the most part run between the intes- 

 tinal wall and the integument, anchoring and making both more firm. These 

 are all rather short. Other strands run lengthwise or obliquely from one part 

 of the outer wall to another, or from a posterior region of the intestine to 

 more anterior parts of the integument. There are also a few fibers reaching 

 from dorsal to ventral points. Taken altogether these groups of strands form 

 a peripheral layer just beneath the skin. 



On the ventral side of the worm most of the muscle cells are connected in 

 some way with the pharynx or mouth and function as an apparatus for either 

 drawing in or protruding the surface. The musculature of the pharynx itself 

 is quite complex. There are comparatively heavy bands of fibers encircling 

 both outer and inner edges of the rosette, while between the heavy petal-like 

 cells very numerous more narrow strands diagonal back and forth. The circu- 

 lar bands act as sphincters and by the force of their contraction cause the 

 pharynx to work as a rubber bulb exerting power sufficient to dislodge very solid 

 particles of food material. 



