24 ILLINOIS BIOLOGICAL MONOGRAPHS [242 



present. The interpretation of Braun, Looss, Ward and others is indeed 

 well exemplified in many cases where the anterior region is telescoped 

 posteriad over the opened anterior portion of the pharynx (Fig. 46). 



Records of an oral sucker in this group are few, aside from that 

 found in Cyclocoelum mutabile by Cohn (1902). Wedl (1857) gives an ac- 

 count of the oral sucker in Monostomum lanceolatum as follows: "Der 

 kleine Mundnapf liegt an der Bauchseite des zugeschmalerten Vorder- 

 theiles des Thieres (Fig. 15a) und ist nach riickwarts von einem dickfleisch- 

 igen Bulbus oesophagus (b) begrenzt, * * * ". These with the account 

 of the writer (Harrah 1921) in which the oral sucker was described 

 in two species of this genus constitute the evidence produced to demon- 

 strate the presence of the oral sucker in this group. 



In the light of the foregoing the question brought out by a long con- 

 tinued controversy remains unsettled. Is the structure termed the phar- 

 ynx by Monticelli (1892) phylogenetically a pharynx or an oral sucker? 

 While Monticelli attempted to prove by the distribution of the anterior 

 nerves that the muscular bulb, or pharynx as he termed it, was a true 

 pharynx, this has not been generally accepted and hence remains a matter 

 of much controversy. In the opinion of the writer the brain commissure 

 which lies distinctly anterior to the pharynx (Fig. 8) can be used as a land 

 mark only and in a different state of contraction might have its relative 

 position changed. The innervation is no doubt distributed to the other 

 anterior structures as well. Although the nerve commissure has the 

 same relative position in the distomes this alone does not prove the phy- 

 logenetic origin of the pharynx, and when a muscular sucking apparatus 

 is found and proof established of such an organ anterior to and adjoining 

 the pharynx, as predicted by Cohn (1904), then and only then can these 

 organs be safely designated as oral sucker and pharynx. 



In this study the writer has examined more than one hundred speci- 

 mens of the genus Cyclocoelum Brandes comprising at least fifteen different 

 species. In this material different conditions are found. In Cyclocoelum 

 obliquum Harrah 1921, Cyclocoelum halli nov. spec. Cyclocoelum obscurum 

 (Leidy), Cyclocoelum triangularum nov. spec, a very weak and scarcely 

 distinct oral sucker is present. The concentration of tissue is scarcely 

 discernible except under the best optical conditions and even in sections 

 there appears only a concentration of tissue at this point (Figs. 36-42). 

 The outer circular band or sheath is found to be very light and not a con- 

 tinuous band as in Cyclocoelum elongatum. The above condition has 

 been found to obtain in Cyclocoelum problemalicum Stoss. and Cyclocoelum 

 tringae (Brandes). In these species the sucker musculature is easily over- 

 looked and when not taken into account the mouth opening agrees well 

 with the structure so clearly described by von Siebold (1835) and van Bene- 

 den (1858) and that named prepharynx by Monticelli (1892). On the 



