223] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 29 



in part or entire, and the Stenostoma will often attempt particles entirely too 

 large to be managed. Spiny, rough, or even slightly irregular bits are almost 

 never touched altho they may be small and easily captured. That very large 

 bodies can be passed thru the slender pharynx is evident, if the more or less 

 constant dilation and contraction be noticed. When the intestinal valve is 

 closed and the long pharynx is collapsed, it appears as only a narrow Hne 

 running back from the buccal indentation. At various intervals, often rather 

 suddenly, the mouth is opened, very wide, so that its diameter is nearly that 

 of the body, then immediately the pharynx is dilated almost to a sphere, dis- 

 placing the parenchyma of the surrounding region and giving almost a globular 

 shape to the head. It is this elasticity of the walls and enveloping parenchyma 

 as well as the heavy muscular contraction which give the possibility of ex- 

 tended variation both in size and shape. The intestinal opening is also capable 

 of enormous distention. The extreme flexibility and lack of cell intimacy have 

 been mentioned as one of the important characters of the phylum, and the de- 

 velopment of such a character seems to have reached its height in this species, 

 a fact which to some extent explains the variation in the size of the food taken. 

 Another striking detail of appearance is the difference in shape, size, and 

 number of zooids among individuals not only of several ponds but those from 

 one part of a single pond. This difference in appearance is the result of the 

 formation of zooid chains, and all stages of growth are to be found present at 

 almost any given time. In TocaUties where conditions of food, oxygen, and 

 temperature are ideal or nearly so, the chains of zooids form rapidly, and the 

 segments cut off are small and blunt at first, altho they elongate very soon. 

 Child (1902) worked out very thoroly the history of regeneration and the 

 stages thru which the segments pass before they are themselves ready to divide. 

 It is commonly known that almost immediately upon being separated, the 

 segments invariably attach themselves to some convenient substratum. The 

 subsequent shape. Child says, is the result of this habit, and the "elongation 

 of the body can be prevented by preventing the animals from attaching them- 

 selves. " His final conclusion is of especial interest. "Due to attachm.ent of 

 animals by the tail, and to mechanical tension caused by cUiary action, the 

 form of vegetating pieces is changed; it is a mechanical phenomenon and not 

 the effect of stimuli. " The truncated, anterior zooid after the cutting off of 

 the others, is a conspicuous shape very frequently seen. It can never attach 

 itself and so rather aimlessly swims about stiff and awkward until the rounded 

 short posterior end begins to become normal. In no other type is the shape so 

 entirely dependent upon physiological condition or relative age. The number 

 of zooids for this species is generally two, altho longer chains are often to be 

 seen. This is due to the fact that fission planes are rarely interpolated between 

 others and that the first division takes place before a second zone of division 

 begins. This is quite the opposite condition from that in Microstoma, where, 

 when fission planes are laid down at all, they very closely follow each other. 

 It may be said that in Stenostoma the asexual budding is not to be correlated 



