DISTRIBUTION OF CLINOSTOMUM MARGINATUM. 359 



this pose at the moment of fixation. The region of the body 

 chiefly active in the production of this pose is the part anterior 

 to the ventral sucker. Ordinarily this part of the body has the 

 shape of an obliquely truncated cylinder. In assuming this 

 pose the worm draws the end of the body back into the interior, 

 the side walls being thicker and acting as the rim of a sucker 

 projecting considerably beyond the level of the center. This 

 inversion of the oral end of the body is brought about by. the 

 contraction of fibres of the parenchyma muscle system which 

 run longitudinally in the body. Some of these fibres are shown 

 in Fig. 7. Their contraction pulls the center of the oral end back 

 and the margin is left projecting. If this action were to take 

 place at a moment when the oral end was in contact with a soft 

 surface, for example, such a surface as the mucous coat of the 

 stomach or throat of the heron, then the soft substance of the 

 host would be sucked into the cavity of this sucker and a powerful 

 adhesion of the parasite to the host would result. A worm liber- 

 ated naturally from the cyst would meet conditions which would 

 furnish responses to this movement, so that instead of being 

 merely a momentary pose it would be useful and so continued. 

 On the other hand, in the case of the artificially liberated worm 

 such stimuli being absent the worm returns to its customary 

 form. The sucker thus formed is additional to the two usual 

 trematode suckers. We may suppose that in as large a parasite 

 as Clinostomum and one which lives in such an exposed place 

 as the gullet where large masses of food are pressed against 

 it additional adhesion would be needed to protect it against being 

 dislodged by the pressure of the food as it is being swallowed. 

 This striking movement had thus evidently a purposive char- 

 acter, as can be seen if we take into account the environment 

 for which it has been developed. 



The "swimming pose" is shown in Fig. 9. In this case the 

 body is made broader and flatter than usual, the ventral surface 

 becoming somewhat concave. The margins of the posterior 

 part of the body are reduced to fine sharp lines like fins which 

 do not extend into the front part of the animal but crossing it 

 ventrally converge toward the ventral sucker. The line shown 

 in the figure is not visible in any specimens of the worm after 

 preservation and is only a momentary structure. The flattened 



