192 HENRY LESLIE OSBORN 



to the great mobility of the animal, but the series is too regular 

 to be wholly due to mere differences of degree of extension of 

 animals of a constant length and indicates also the existence 

 here of a length variation such as is common in all animal groups. 

 They furnish further an indication that the encysted worms, 

 which are slightly younger, are smaller than the heron worms. 

 The average length of the eleven specimens from the bass is only 

 4.1 mm. while that of the sixteen worms from the heron, fixed 

 with the same reagent, is 4.59 mm. It is interesting to note the 

 larger figures for the chromic acid material. The average length 

 of these individuals is 6.77 mm. 



The form of the anterior end of the body is remarkable. In 

 many distomes the walls of the body converge anteriorly and meet 

 at the mouth, here they run parallel until they intersect the mar- 

 gin of a peculiar area, the oral field, which closes the anterior end 

 of the body. In an animal in which the oral field is in the resting 

 position, as in fig. 4, it is oblique to the axis of the body, with the 

 dorsal side projecting somewhat beyond the ventral. It is this 

 obliquity to which Leidy's generic name alludes. Fig. 1 shows 

 the margin of the oral field where it meets the side wall. Often 

 there is a slight depression in the margin of the field at this point. 

 In fig. 5 the field is retracted, a very frequent act of the living 

 animal; this section is from a specimen which was caught by the 

 fixing reagent in this act. A fuller account of the oral field struc- 

 ture will be given later. 



The ventral sucker (figs. Iw, 3 C, and 4) is a very conspicuous 

 organ, both in the whole animal and in sections and is much larger 

 than the oral sucker. In all living and preserved animals which 

 I have seen it is entirely enclosed within the contour of the body. 

 Its opening is always very distinctly visible and is usually tri- 

 angular, with one of the equal sides anterior and the apex pos- 

 terior. In some cases, however, as in fig. 1, the three angles are 

 rounded, or the opening (as in Linton, '98, pL, 44, fig. 6) maj' be 

 circular or even almost square (Wright, '79). The sucker has 

 a length of 0.7 mm. and the same width and measures 0.4 mm. 

 dorsoventrally. It is about half as thick as the body and reduces 

 its space very much, as shown in fig. 4 C. The reason foi- the 



