239] NORTH AMERICAN MONOSTOMES 21 



specimens of Heronimus chelydrae MacCallum (1902) both living and pre- 

 served not a single instance has been found. For these reasons the writer 

 feels safe in saying that these pits are characteristic of the living animal 

 and are only emphasized by the state of contraction at the time of preser- 

 vation. The various worms being fixed in different states of contraction 

 would consequently show these pits more conspicuously in the more con- 

 tracted specimens. 



The body wall is composed of at least five layers. From outside inward 

 they are as follows, cuticula, basement membrane, circular muscle, longi- 

 tudinal muscle, and epithelium (Fig. 25). These compose what is com- 

 monly known as the dermomuscular sac. The disposition of the parts of 

 the dermomuscular sac of this group agrees in most respects with the inter- 

 pretation of Monticelli (1888) and Blochmann (1896). It differs however 

 from the observation of Fuhrmann (1904) in which he says that the body 

 musculature is differentiated into outer longitudinal, inner circular muscle 

 layers and inside of this the layer of bands of diagonal muscles, in that 

 the outer muscle layer is formed by circular muscles (Fig. 25). The single 

 statement of Fuhrmann mentioned above is so strikingly different from 

 all comprehensive works on this subject that the writer is lead to believe 

 that it is a lapsus calami and that in reality the muscle layers of Bothrio- 

 gaster variolaris are identical with those of other trematodes. 



Zeder (1803) states that these worms have a single sucker on the for- 

 ward end. His description of this organ is scanty and lacks the points 

 which distinguish the sucker from the pharynx so that one is lead to believe 

 in the light of present knowledge that he interpreted the pharynx in the 

 Cyclocoelidae to be the same as the sucker in the Notocotylidae. Von 

 Siebold (1835), the first to give a clear account of the anatomy of the 

 Monostomidae in his description of Cyclocoelum {Monostomum) mutabile 

 (Zeder), speaks of the mouth as a transverse oral opening leading to a 

 funnel shaped canal which narrows gradually posteriorly and terminates 

 in the so-called pharynx. No trace of a sucking organ was observed by 

 this author. 



Following this Van Beneden (1858) referred to the above work fre- 

 quently but stated that the monostomes have only a mouth sucker situated 

 in the anterior region. In another paragraph of the same work he speaks 

 of the digestive system of Trematodes as showing generally an anterior 

 sucker in the bottom of which is situated the mouth. This he says opens 

 into a second enlargement similar to the preceding sucker, the pharyngeal 

 bulb. In his figures of Cyclocoelum {Monostomum) mutabile (Zeder) the 

 structure termed pharyngeal bulb above is indexed as buccal bulb. These 

 show the pharyngeal bulb with no anterior sucking musculature surround- 

 ing the mouth opening. In a later paper (1861) the same author describing 

 again this same species spoke of the bulb and the region preceding it which 



