1 8 



THE TURDELLARIA 



majority of the Rhabdocoelida under some form or another, (b} 

 Ph. plicatus, in which the sheath is absent, is characteristically 

 developed in the other two orders ; it is really an acrecbolic intro- 

 vert and occurs under two forms, which perhaps represent two 

 stages in development. In the Polycladida the pharyngeal sac is 

 of considerable diameter, with its axis at right angles to that of 

 the intestine ; the pharynx has the form of a freely projecting 

 muscular fold arising from the circumference of the sac, than 

 which it is frequently larger, and is, therefore, much folded when 



at rest ; when brought into use this muscular organ is protruded 

 through the mouth, and is then spread out so as to envelop the 

 prey (Fig. VI. 9, 12). In a few of the Polyclads the axis of the 

 pharyngeal sac shifts so as to lie nearly parallel to that of the 

 intestine instead of at right angles to it ; and the entrance to the 

 intestine is no longer directly over the oral aperture ; the muscular 

 fold now loses its irregular shape, and its base becoming smaller it 

 forms a muscular cylindrical tube ; this is the form of pharynx 

 characteristic of the Tricladida (Fig. VI. 10, 13). Glands, more 

 or less abundant, exist in or around the pharynx, pouring their 



