253] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 59 



form, much as plant cells are made resistant by their turgidity. Then, too, 

 the soft tissues are held in place and prevented from crowding one another 

 so that their relative positions remain intact. 



As well as serving for purposes of support, this most primitive of all the 

 tissues also protects. It is very spongy, being capable of undergoing a great 

 amount of compression, as for example when the animal squeezes thru extreme- 

 ly small openings or when it bends or twists so that some one portion of the 

 body is subjected to especial strain. The great amount of elasticity is also 

 noticeable and causes the shape to vary. Thus, when the intestine is large 

 and its wall thick, or when, during the period of sexual maturity, the repro- 

 ductive organs occupy a great amount of space, the parenchyma is so com- 

 pressed as to seem almost lacking. On the other hand, when the digestive 

 cavity is empty and shrunken the whole body mass appears to be composed 

 of parenchyma, the cells of other sorts being insignificant in comparison. In 

 the case of injury, also, this valuable layer plays the main part. Forming 

 scar tissues and furnishing cells for regenerative purposes, it seems embryonic 

 in its ability to develop in various directions. It seems not far from correct 

 to sum up the foregoing characters by comparing the parenchyma with the 

 embryonic tissue of other forms. 



DIGESTIVE SYSTEM 



The digestive system is more complex than that found in most of the other 

 groups of this class. It consists of a very well-defined pharynx opening into a 

 rather simply shaped, sac-Hke intestinal cavity which occupies the greater 

 part of the body space. So conspicuous is the whole structure that the general 

 appearance of the animal depends entirely upon physiological condition. 

 While all the other tissues are transparent and clear, the alimentary tract 

 alone shows heavy and dark, giving the only suggestion of solidity, and with- 

 out it the individuals would seem altogether ephemeral. 



Situated in the middle of the body on the ventral side is the pharynx, 

 which viewed from above is a large, sharply marked rosette. It is in reahty 

 an inverted cone or funnel with the small round mouth at the tip, opening 

 down. The rosette is never entirely flattened, but slopes upward and outward 

 to its connection with the intestinal wall. This connection is made with the 

 very outermost edge of the pharynx, just above the large circular muscle band 

 already mentioned. The lower, mouth-end of the cone is held in place by its 

 attachment with the epidermis and by means of various muscle strands which 

 also anchor it to portions of the ventral surface. There is no sharp line of 

 demarcation between the external epidermis and the epitheHum lining the 

 rosette, so that the mouth is bounded only by the circular sphincter muscle. 

 The pharynx wall, itself, is thick and heavy, due mainly to the size of the 

 petal-like cells which, together with muscles which control them, give the 

 characteristic shape. There are from seven to ten of these wedge-shaped 

 structures arranged with the smaller end pointing down toward the mouth. 



