and chiefly in the clamps. A typical clamp of Hexostoma has the shape 

 - 



In addition to the literary data we have utilized our own relatively sparse 

 materials on Hexostoma grossum Goto, 1894. 



of an oval sucker stretched longitudinally in reference to the body but in 

 transverse direction of the clamp (Fig, 299). Thus the half of each clamp 

 lying toward the middle axis of the body of the worm corresponds to the 

 anterior valve of typical clamp of Oligoncholnea and the opposite one --to 

 the posterior one. The "clamp" of Hexostoma (Fig. 299, A) represents a 

 sucker which has a weak cup-shaped fornn which acts, in contrast to all 

 remaining Oligoncholnea, precisely as a sucker. One should only take into 

 consideration the secondary nature of this "sucker" which is substantiated by 

 its morphology. It is flatter than the usual sucker and it is divided into 2 

 depressions right and left in connection with the presence of a more or less 

 strong thickening of the middle chitinous piece along the middle line. Although 

 the latter is usually described as having an irregular form [see the des- 

 cription and drawings of Goto (Goto, 1894)], actually it represents, judging 

 by our preparations, a fully symmetrical formation with insignificant 

 differences in the details of right and left sides. The middle piece (Fig. 

 299, B) lies with its long axis along the longitudinal axis of the clamp so 

 that its anterior half is located in the anterior part of the clamp and the 

 posterior- -in the posterior and hence in the shape of a flat, cake-shaped 

 plate folded in half longitudinally and thus are formed the right and left 

 wings. The rib of the plate faces edgewise toward the ventral side with 

 both free edges toward the dorsal. The rib is somewhat thickened and 

 compressed in the anteroposterior direction , in connection with which 

 during observation of preparations it gives the impression of an x-shaped 

 figure which is described by a number of authors (Price, 1943; Sproston 

 1946, and others). Actually, as is clear from what has been said, this is p. 422 

 a purely optical phenomenon. The right and left wings are of ear-shaped 

 form with more or less even edges. The middle part of each wing is 

 pierced by 3-4 openings of irregular form analogous to the ones of the 

 bottom of the plate of the clamp of Mazocraeidae, Apparently because 

 of the necessity of the attachment of the plate from the ventral side it 

 forms thickening in the central part which unites both wings and has one 

 large opening in the center. This opening serves, as we succeeded in 

 observing during microdissection for the attachment of the powerful 

 structures of the musculature. It seems to us that the middle piece of 

 the clamp represents the remaining part of the capsule of the clamp of 

 Mazocraeidae and most probably basically the plate which forms the bottom. 

 This is even more likely since the locations of both formations fully 

 correspond to each other. As regards both edge plates of the clamp of 

 Hexostoma (Fig. 299, C and D), they have a fully determined but not 

 symmetrical form. At the same time, asymmetry is both longitudinal and 



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