24 ILLIXOIS BIOLOGICAL MOKOGRAPHS [304 



similar arrangement has been described in Calicotyle, Axine, Nitzschia, 

 Tristomum, Octobothrium, Temnocephala, Microcotyle, Octocotyle, and 

 Monocotyle. In Diclidophora Goto (1894) described an additional layer 

 of longitudinal fibers between the circular and diagonal layers. He states 

 that in Onchocotyle and Hexacotyle the circular fibers seem to be en- 

 tirely lacking. In the genus Polystoma there are strong sets of longitudinal 

 fibers near the median line on the ventral side of the body. They could 

 be traced anteriad only to the testis. Posteriad they pass into the caudal 

 disc and together with fibers from the body wall are inserted on the sides 

 and in the bases of the bothria. Muscle strands from both sides of the 

 body pass to each of the suckers (Fig. 29) and smaller groups of fibers 

 from each sucker to each of the others. In addition to the dorso-veutral 

 muscles which extend between various points of the body wall, there are 

 other fibers from the body wall to the internal organs. 



Mesenchyma. — The mesenchymal tissue of the body does not sliow 

 a differentiation into ectoparenchyma and endoparenchyma as described 

 by Brandes (1892) and other writers; it is not of a uniform character, 

 but presents differences in appearance at different points in the same 

 specimen. It may take the form of compact cellular tissue, or of vacuo- 

 lated cells, or there may be large vacuoles apparently between cells, or 

 the cellular structure may be entirely lacking, there being only a reticu- 

 lum of fibrous tissue. The parenchyma is traversed by many muscle 

 strands, and the dorsal and lateral regions are occupied by the enormous- 

 ly developed vitellaria (Figs. 19, 23). 



Alimentary System. — The digestive apparatus consists of a terminal 

 anterior or oral sucker, a pharynx, a short esophagus and a bifurcate in- 

 testine. The oral sucker (Fig. 6) is not fully homologous with that of 

 the distomes. There is no external limiting membrane, branched muscle 

 fibers passing from the inside lining of the sucker to the body wall. 

 Posteriorly it is limited and separated from the body parenchyma by 

 special strands of fibers which pass from the body wall to the wall of the 

 digestive tube and are attached there just anterior to the pharynx. A 

 contraction of these fibers causes the constriction between the anterior 

 sucker and the body parenehj^ma which is sometimes seen. Longitudinal 

 muscle fibers from the body parenchyma penetrate this posterior boun- 

 dry of the anterior sucker and pass to the wall of the sucker. Annular 

 muscles, situated just inside the cuticular lining, pass around the sucker 

 from side to side. Situated among the muscle fibers there are large 

 secretory cells. Johnston described the structure as a weakly developed 

 or incipient oraj sucker. The anterior sucker, pharynx, and esophagus 

 are lined with cuticula continuous with that of the external surface of 

 the body. 



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