26 ILLINOIS BIOLOGICAL MONOGRAPHS [244 



thus leaving some 150 to 200 ju between the oral sucker and the pharynx. 

 This portion is termed the prepharynx (Fig. 29). 



As will be seen by the description which is to follow later the structure 

 of the pharynx is practically identical in many of the species of this and 

 other families and therefore must necessarily have developed from a 

 similar tissue in a similar manner. This being the case there remains the 

 functional differentiation which may come about in the absence of a well 

 developed oral sucking structure. No doubt the pharynx aids the weak 

 sucker in drawing in food and it is highly probable that the suction pro- 

 duced by these heavy muscular walls is indeed very great. As was stated 

 earlier in this discussion the position of the oral sucking apparatus relative 

 to the pharynx in all those species which do not possess a strong oral 

 sucking apparatus strongly indicates this view. 



As was stated before the muscular bulb, or pharynx of Monticelli, is 

 identical in structure and has no doubt arisen in the same manner in all the 

 species of this family. Even though it may function as a sucking organ in 

 some instances this structure has not been modified and does not appear 

 to be typical sucker tissue as it has been described in other trematodes. 

 If, however, this muscular organ is present in one or more species of this 

 genus at the same time with a distinct anterior musculature which is 

 typical of that tissue as found in other groups of trematodes and is con- 

 stant in these species, then one is justified in designating the anterior 

 musculature a sucking organ or oral sucker and the musculature posterior 

 to it a pharynx as it was termed by Monticelli (1892). 



Immediately posterior and dorsal to the prepharynx is a muscular 

 structure developed around the wall of the posterior prepharynx or an- 

 terior esophagus. On first observation the pharynx appears as two bean- 

 shaped halves lying one on either side of the anterior portion of the 

 esophagus. On more careful study, however, it is found to consist of heavy 

 muscular halves bound together on the edges by smaller bands of fibers 

 so that in the true cross section it appears as a cylinder with an elongated 

 dorso-ventral slit passing through it. 



The pharynx is variable in size and form within the species as well as in 

 different species, and may be in some species distinctly elongated while 

 in others it is noticeably expanded laterally and still in others it is spherical. 

 It measures in Cyclocoelum pseudomicrostomum 778/i and is slightly longer 

 than broad. In Cyclocoelum obscurum it is in general spherical and meas- 

 ures 215 to 298 fj,. In Cyclocoelum macrorchis it is distinctly longer than 

 broad and measures 264 to 314ju in length by 198 to 248/x in width. In 

 Cyclocoelum elongalum the pharynx is oblong measuring 264 to 331 fj. in 

 length by 215 to 281 ju in width. While the range of measurements gives 

 a general idea of the size and shape of the pharynx this study has shown 



