54 THE STRUCTURE AND FUNCTION OF THE PROBOSCIS 



The filtering mechanism is situated in the oral sucker or 

 suctorial disc which is described by Lowne (1895, p. 136) as "a 

 fleshy oval disc, deeply cleft at its anterior margin. The edges 

 of the cleft are continuous with the margins of the groove in the 

 theca, and are united as far as the edge of the disc by a remark- 

 able bead and channel joint. The thick edge of one lobe, or 

 labellum, of the disc fits into a corresponding cylindrical channel 

 in the other. The distal or oral surface of the disc is channelled 

 by the well-known pseudo-trachccE. In the centre is a deep 

 longitudinal fissure, which extends into the tubular mouth 

 situated between the labrum and the theca. The proximal or 

 aboral surface of the sucker is convex and covered by setae ; 

 those near its margin are very long and form a fringe." 



EXPLANATION OF PLATE XIII. 



Fig. 1 represents a dissection of a portion of a pseudo-trachea. On the right-hand 

 side the integument of the oral surface of the labellum has been removed so as to 

 show a portion of the pseudo-trachea with the alternate bifid and flattened extremities 

 of the chitinous rings and the membrane lining the interior of the tube stretching 

 between them. On the left-hand lower portion the appearance of the surface 

 integument is represented. Two interbifid grooves leading to their interbifid spaces 

 are shown. Between the spaces are elevated masses, each of which is produced by a 

 fold of the integument enclosing the flattened end of a ring and the extremities of the 

 adjacent forks on each side. In the left-hand upper portion of the diagram is shown 

 the appearance of these structures as seen by transmitted light so as to indicate more 

 clearly the relationship of the integument to the rings. 



Fig. 2 is a photograph ( x 340) of part of the oral surface of the labellum, treated 

 with potash, showing portions of four pseudo-tracheii;. The flattened and bifid 

 extremities of alternate pseudo-tracheal rings are well seen. 



!■'.?• 3 is a photograph ( x 700) of ten consecutive chitinous pseudo-tracheal rings 

 treated with potash and compressed, showing their alternate flattened and bifid 

 extremities. 



Fig. 4 is a photograph ( x 600) of a transverse section of part of the oral surface 

 of a labellum. Four complete pseudo-trachete are included in the section. In each 

 case the chitinous ring and the opening of the longitudinal fissure is very distinct. 

 It happens that in each case the bifid extremity is situated on the left side of the 

 pseudo-trachea. The point of bifurcation is indicated by a dark spot above which 

 the forks are curved inwards. From the point of bifurcation a distinct line, which 

 represents the reflection of the cuticle at the base of the interbifid groove, passes 

 obliquely upwards and outwards to the surface of the integument (see Fig. 6). 



Fig. 5 is a photograph ( x 600) of a longitudinal section through a pseudo-trachea 

 slightly to one side of the median line. The interbifid spaces and the manner in 

 which the integument passes over and binds together the flattened extremities of the 

 rings and the contiguous forks of the adjacent rings on each side can be clearly seen. 

 (From Graha.m-Sm'nh, yournai 0/ I/)'£^iene, 191 1-) 



