3011 XORTH AMERICAX POLYSTOMIDAE—STUSKARD 21 



Sections show that the cuticular lining of the sucker is folded outward 

 against the convex wall with which it is fused, thus interrupting the con- 

 tinuity of the fibrous wall (Fig. 35). The two sides of this invagiuated 

 cuticular sac or ring are fused at regular intervals, leaving small pockets 

 alternating with the places of fusion. These small openings in the cuti- 

 cular band are conspicuous by reason of their different refractive index 

 and show very plainly with a dark field illumination as the square or 

 rectangular sections with the circular areas in the center (Fig. 3i). 

 There is apparently no relation between the number of these sections in 

 the cuticular band and the number of cuticular thickenings which serve 

 as supports of the external section. 



The middle section of the sucker extends basally from the previously 

 described cuticular band to a somewhat similar evagination of the cuti- 

 cular lining into the wall of the sucker, but this evagination does not ex- 

 tend to the external cuticular covering of the sucker and onlj- partially 

 divides the fibrous wall. This middle or intermediate portion of the 

 sucker is supported by thickenings of the cuticular lining, processes that 

 extend peripherally from the cuticular band which passes around the 

 sucker at its base. These supporting ridges are not arranged at regular 

 intervals and they are much fewer in number than the cuticular rods 

 which support the external section. They are often branched, tho not 

 more than a single bifurcation was observed. 



The basal portion of the sucker is circular, similar in structure to the 

 portions previously described ; it has internal and external limiting mem- 

 branes with fibers extending between. At its center the cuticular and 

 fibrous wall is interrupted and there is the structure described by John- 

 son (1912) as the connective tissue plug, which appears as a central disc 

 or button, and to which the retractor muscles are attached. This central 

 disc has thickened cuticular edges and bears the larval booklet. Figure 

 44 illustrates the method of operation of the suckers. Muscles are at- 

 tached to the external wall of the distal and intermediate portions of the 

 sucker and the contraction of these muscles retracts the two external 

 zones, with the accompanying protrusion of the basal part. Whether the 

 small hooks at the bases of the suckers are functional is doubtful. As 

 previously described, the cuticular supports do not extend quite to the 

 external margin of the sucker, leaving a soft plastic edge which can be 

 applied all the waj- around even on an irregular surface. With the con- 

 traction of the muscles attached to the basal disc, a vacuum is produced 

 and forms a powerful means of adhesion. Since the walls of the sucker 

 are not contractile and the suckers vary only slightlj' in size in a single 

 species, the size of the suckers has been used by the writer as a character 

 for determining specific identity. 



