TABLE OF CONTENTS 



Introduction 7 



Some new methods of nematode technique 7 



Symmetry and structure of the head region 13 



General considerations 13 



Type form of the primitive nematode 13 



Alterations in bilaterality of primitive type 16 



Primitive orientation of nematodes 16 



Definition of the primitive nematode 18 



Cephalic structure in free-living nematodes 20 



Symmetrical type of the esophagus 20 



Pharyngeal modifications 22 



Oral structures among nematodes 24 



The structurally simple form 24 



The primitive form 24 



Modifications by division and fusion of structural elements 25 



Modifications by loss of parts 28 



Considerations of symmetry in the head region 29 



Primitive and fundamental symmetry 29 



Radial symmetry 30 



Disymmetry 30 



Asymmetry 30 



Cephalic structure in parasitic nematodes 31 



Symmetrical type of the esophagus 31 



Cephalic modifications and relation to habitat 31 



Pharyngeal modifications 33 



Oral structure and symmetry 33 



Simple forms 33 



Disymmetrical forms from three-lipped ones 34 



Other disymmetrical forms simulating jaws 34 



Variations arising from number of lips and capsules 35 



Ciliation among nematodes 39 



Former views and present data 39 



Significance of ciliation 41 



Discussion and Conclusions 42 



List of free-living species cited 44 



List of parasitic species mentioned 44 



Bibliography 47 



Explanation of Plates 56 



