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University of California Publications. 



[Zoology 



the substratum, the membranellae are now the strongest factors 

 for locomotion and consequently influence the reaction. When 

 the anterior end is reduced in size to that of piece A, fig. 9, it 

 describes smaller circles than when the piece is larger (piece A, 

 fig. 8) showing that the membranellae have a relatively greater 

 influence when the directive effect of the posterior end is lost. 



Fig. 7. — During gliding Piece A pursued a straight course. 



Fig. 8. — During gliding Piece B pursued a curved path as normal individ- 

 uals do. 



Fig. 9. — Piece A is smaller than Piece A, fig. 8, and shows more deviation 

 to the right. 



Sumniary. — During the gliding movement the anterior end 

 is depressed, the left lip is extended and the buccal groove ex- 

 posed on the left side. The entire ventral surface is close to the 

 bottom with the exception that in movement on the surface film 

 the adoral zone is the only part in actual contact. The posterior 

 end is bent towards the left and held very close to the bottom, 

 giving the appearance of dragging. The membranellae together 

 Avith the cilia are the organs of locomotion. Gliding on the sur- 

 face film is faster than on the bottom of the acpiarium. The 

 direction of the gliding is dependent on the shape of the body. 

 The bending of the posterior end toward the left acts as a rudder 

 and causes the animal to turn towards the left. 



