28 ILLINOIS BIOLOGICAL MONOGRAPHS [246 



weight on the exact length of the esophagus as a specific characteristic. 

 In fact in some cases where the toto specimen showed the esophagus to be 

 very short, sections of the same specimen gave evidence of a relatively 

 long but much folded tube due no doubt to contraction. 



In the case of Cyclocoelum obscurutn stated above the length varies 

 from 298 to 662ju making an average for the eleven specimens of 419^. 

 Under the conditions mentioned the average does not represent the actual 

 length of the esophagus. It was noted from a study of this collection that 

 the esophagus of one individual was more than twice as long as that of 

 another. However, the maximum length of the esophagus is not repre- 

 sented by the maximum measurements secured since in no case has the 

 writer observed the esophagus when it could be said to form a straight 

 line from the oral sucker to the pharynx. The normal condition is repre- 

 sented in figure 29 where the esophagus arises at the dorsal portion of the 

 pharynx and from there takes a winding course both laterally and dorso- 

 ventrally to the intestinal bifurcation into which it empties directly from 

 the ventral side of the latter organ. The esophagus bifurcates at the 

 posterior portion of the first body sixth to form the intestinal crura. One 

 branch passing to the right, the other to the left side of the body, thus 

 forming an arch which lies in the anterior one-sixth of the body. The 

 crura extend from this parallel to the body wall, being separated from it 

 only by the vitellaria, to the extreme posterior end where they anastomose 

 forming a posterior arch. This is separated from the posterior body wall 

 by the excretory bladder. The crura are usually large with a large lumen. 

 However, in some cases they are extremely irregular and show in a few 

 instances more or less distinct pouches which in some individuals appear 

 as distinct diverticula and in fact are as strongly exemplified as some of 

 those shown by Stossich (1902) and Kossack (1911) for the genus Typhlo- 

 coelum. This feature, however, is not constant for any species of the genus 

 Cyclocoelum observed by the writer and is probably due to the pressure 

 produced by expanded uterine loops which fill out the space between the 

 crura and in many instances reach over the crura. Anterior to the intes- 

 tinal bifurcation in Cyclocoelum macrorchis and Cyclocoelum obscurum 

 there is an evagination which makes a pronounced undivided neck to the 

 crura into which the esophagus opens. This appears in every specimen of 

 the fifty-six in the two collections. This condition is in decided contrast to 

 the other species studied. In these specimens, however, the intestinal 

 crura are comparatively empty and show a generally relaxed condition 

 whereas in most of the other specimens the crura are well filled and appar- 

 ently well extended. It is probable in view of the relaxed and empty 

 condition of the crura of the former species that the neck of the bifurcation 

 is the result of the condition rather than a constant character for the 

 species. 



