245] NORTH AMERICAN MONOSTOMES 27 



that th,e individual which has the longest pharynx is not always the one 

 which has the narrowest one and vice versa. 



As stated above the size and form of the pharynx is not constant as 

 might be indicated by an average or range of measurements. Cydocoelum 

 ohscurum shows eight instances in which the pharynx is round, two with a 

 greater width than length, one with a greater length than width; this gives 

 an average for eleven specimens of 230/i in length and 231 )u in width, 

 agreeing closely with the record that eight of the eleven cases cited show 

 the pharynx to be spherical. From the range one would infer from the 

 maximums that the pharynx is spherical but from the minimums of less 

 transverse diameter than longitudinal. In Cydocoelum macrorchis and 

 Cydocoelum elongatum every specimen measured shows for the pharynx 

 a greater length than width while in Cydocoelum pseudomicrostomum and 

 Cydocoelum ohscurum some were spherical while the majority have a 

 greater length than width. From this comparison one can see readily that 

 the pharynx possesses such variability in size and form in this genus that 

 it can be termed at best only spheroidal. 



The musculature of the pharynx is, as stated previously, similar in 

 all the species of this genus and consists of numerous fibers bound into bun- 

 dles that are so interlaced as to make a very powerful organ and in the 

 absence of the strong oral sucker it is quite probable that it serves as a 

 sucking organ as was stated earlier in this work. 



The musculature is quite characteristic of this organ as it has been 

 described in other trematodes and consists of circular, radial and longi- 

 tudinal fibers. The circular muscles are most numerous and constitute 

 approximately 75 percent of the entire structure. They are pierced by 

 bundles of radial muscles which have their origin in the outer layer of 

 circular muscles and their insertion in the inner layer of the same muscles 

 whose fibers intertwine all the muscles of the circular and radial type, par- 

 ticularly at their origin and insertion. 



In most cases this muscular bulb stands with the anterior end open 

 thus forming a continuation of the funnel shaped mouth opening to the 

 posterior end of the pharynx which in all instances observed by the writer 

 is closed just anterior to the esophagus that leads caudad from this organ. 



The esophagus is a thin walled tube of varied length. In Cydocoelum 

 elongatum it measures 347 to 463/x; in Cydocoelum ohscurum 331 to 662)u; 

 in Cydocoelum halli 483ju on the average. The wide range of variation in 

 length is due to the fact that the esophagus takes an S-shape (Fig. 29) 

 which is no doubt due to a state of partial contraction taken at fixation. 

 This condition makes it impossible to secure the exact length of this organ. 

 Because of the different states of contraction the esophagus is more sinuous 

 in some specimens than in others and consequently shows a much greater 

 variation in length. In view of this fact the writer can place very little 



