243] NORTH AMERICAN MONOSTOMES 25 



other hand in Cyclocoelum pseudomicrostomum an intermediate condition 

 is found. On first observation the mouth opening appears like the one 

 just described; however, on more careful scrutiny a light but well formed 

 sucker is discernible. In this case, however, the longitudinal and oblique 

 muscles are actually increased immediately surrounding the buccal duct 

 which leads inward toward the very muscular pharynx (Fig. 43). From 

 this more or less muscular wall, radial muscles extend outward having their 

 origin in a weak band of circular muscles. This outside covering of the 

 sucker is held in place by the same sort of transverse muscles as in the 

 distomes. Were this the only case found one could perhaps accept Cohn 

 (1902:715) who has observed what he terms a rudimentary mouth sucker 

 in Cyclocoelum mutabile (Zeder). 



The maximum condition observed by the writer was that found in 

 Cyclocoelum elongatum, in which posterior to the opening of the mouth, 

 which is downward as before, is seen a large weak oral sucker scarcely vis- 

 ible in toto preparations. It is from one-third to one-half larger than the 

 pharynx posterior to it and extends from the extreme anterior of the 

 animal to well over the anterior portion of the pharynx. It measures 

 314/z in length by 463/x in width. The musculature is much less strongly 

 developed than that of the pharynx and consists of an outer circular layer 

 connected by radial, longitudinal, and oblique muscles to a much heavier 

 inner circular band which forms the muscular walls of the mouth (Figs. 

 44, 45, 47, 48). This muscular body is suspended by much lighter strands 

 of transverse muscle having their origin in the musculature of the body 

 wall and their insertion at times in the outer circular band of muscles 

 covering the sucking musculature and again in the radial muscles of the 

 sucker itself. In general the position of this sucker is such that it opens 

 downward but suspended as it is a slight contraction of the dorsal sus- 

 pensory muscles and at the same time a relaxation of the ventral ones 

 could easily give to the sucker a different position so that its aspect would 

 be changed from that of its true antero-ventral one (Fig. 29). 



In all specimens studied the writer has found evidence of the oral 

 sucking mechanism and believes it to be a universal character in this 

 family. In a study of Haematotrephus similis Stossich a sucker almost 

 as heavy as that of Cyclocoelum elongatum was found (Fig. 28), and in the 

 former species the sucker is considerably more prominent than in Cyclo- 

 coelum pseudomicrostomum and Cyclocoelum mutabile. Cyclocoelum micro- 

 stomum could not be obtained for study of this feature. 



Following the oral sucker is a thin walled, lightly muscular tube extend- 

 ing posteriad and ventrad to the pharynx and opening into it on the 

 ventral side. The writer believes this to be a condition due to the state of 

 contraction at the time of preservation and that in a fully extended speci- 

 men the oral opening would enter the pharynx from the anterior face, 



