12 WRIGHT AND MACALLUM. [Vol. I. 



It is interesting to note that here, as in Polystomum, each 

 sucker is developed around a hooklet. In the two European 

 species of Polystomum (/*. integerriimnn and occllatuni) different 

 booklets serve as starting-points for the suckers, if we may 

 judge from the figures of Zeller and v. Willemoes-Suhm ; ^ but in 

 P. od/o7i£-u7n, described by the seniorauthor (loc. cit.), from the 

 urinary bladder of the Musk Turtle {^AromocJielys odoratiis) the 

 arrangement of the booklets is similar to that in P. integerrimnm. 

 The booklets of Sphyranura are persistent structures; we have 

 never observed them to be absent, even in the largest examples, 

 and are inclined to doubt that they are ever larval organs, as 

 asserted by v. Willemoes-Suhm (loc. cit., p. 37) for Poly- 

 stomum. 



In examining the suckers from the ventral aspect the observer 

 is struck by the reticulation of the surface of the investing mem- 

 brane (Fig. 4) ; by the subdivision of the cavity into concentric 

 zones, and by the thick, strong, doubly contoured line which 

 surrounds the hooklet. These appearances are explained by 

 examination of a vertical section through the centre of a sucker. 

 (Fig. 3.) From such it is seen that the substance of the sucker 

 is bounded on all sides by a membrane continuous in the cavity 

 of the sucker with the investing membrane, and resembling that 

 in appearance, although much thinner, where it bounds the con- 

 vexity of the sucker. Instead of the substance of the sucker 

 being formed of muscular fibres disposed in three directions, 

 and capable of modifying the shape of the cavity, as in the Dis- 

 tomes, it is not possessed of contractility in Sphyranura (and 

 probably in Polystomum), and is formed of prismatic fibres, 

 rather of a supportive than of a muscular character, arranged 

 perpendicularly between the concave and convex limiting mem- 

 branes of the sucker. The fibres measure 2-3 ^i in thickness ; 

 they do not stain, are easily isolated, appear to be homo- 

 geneous, and present between them no nuclei or large cells, such 

 as are characteristic of the Distome sucker. 



The appearance of reticulation in the concavity of the sucker 

 referred to above is due to the superficial ends of these prisms, 

 and is heightened by the dipping down of the investing mem- 

 brane around each. It is to be noted that the investing mem- 

 brane is not only considerably thicker (6 ^) here than else- 

 »Zeit. vviss. Zool., XXII., Tab. I., 9; Tab. III., i. 



