20 ILLINOIS BIOLOGICAL MONOGRAPHS [300 



The suckers are cup shaped (Fig. 34), and in all the species describ- 

 ed in this paper are constructed on an elaborate cuticular framework. 

 According to Zeller the sucker forms as a ridge around a larval booklet 

 and later sinks into the parenchyma, and this method of origin explains 

 the cuticular covering of the external and internal surfaces of the cup. 

 Running across between these cuticular membranes, there are short re- 

 fractive fibers which constitute the mass of the wall of the sucker (Fig. 

 35). Wright and Macallum (1887) describing similar fibers in the walls 

 of the suckers of Sphyranura say, "Instead of the substance of the 

 sucker being formed of muscular fibers disposed in three directions, and 

 capable of modifying the shape of the cavity, as in the distomes, it is not 

 possessed of contractility in Sphyranura (and probably in Polystoma), 

 and is formed of prismatic fibers, rather of a supportive than a muscular 

 character, arranged perpendicularly between the concave and convex 

 limiting membranes of the suckers." Goto (1894) described similar 

 fibers in the suckers of Axine, Microcotyle, Octocotyle, Diclidophora, 

 Hexacotyle, and Onchocotyle and considered them to be more of an elastic 

 than a contractile nature. They are, he states, different from the ordi- 

 nary muscular fibers of the body and from those of the suckers of the 

 Tristomidae and Monocotylidae, as well as from those of the anterior 

 sucker of Onchocotyle, both in optical characters and in reaction toward 

 staining fluids. The structure of the suckers in these forms and their 

 mode of operation are discussed by Goto at considerable length, but as the 

 suckers he described are constructed on a different type of cuticular 

 framework from that present in the genus Polystoma, obviously the type 

 of suctorial action is different. 



In all the species described in this paper, the fibers which form the 

 walls of the posterior suckers are similar to those described by Wright 

 and Macallum and Goto; the cuticular framework is also flexible and 

 elastic, but is of a different type from that described by Goto. In the 

 polystomes investigated by the writer, with the exception of P. integer- 

 rimum, the sucker consists of three sections or zones which may be desig- 

 nated as basal, intermediate, and external or distal (Fig. 36). The ex- 

 ternal part or rim of the sucker is supported by numerous cuticular rods 

 formed by the thickening at regular intervals of the cuticular lining. 

 These rods are bent outward, their curvature maintaining the flare of the 

 rim of the sucker. Distally they terminate just inside the rim of the cup 

 and basally they are continuous with and are processes from a band of 

 cuticula which passes around the sucker and separates the external and 

 intermediate portions. In toto preparations this band appears to be 

 divided into sections that are almost square, each with a circular area in 

 the center that increases and decreases in size as the focus is changed. 



