1171 COMPARATIVE STUDIES ON NEMATODES— HETBERINGTOIf 



SYMMETRY AND STRUCTURE OF THE HEAD REGION 



General Considerations 



Type form of the primitive nematode 



Ward (1917) has utilized for the purpose of grouping nematodes the 

 structure of the anterior end, particularly the oral armature, and has 

 contrasted three terms: lips, jaws, and capsule, each referring to a typical 

 oral organization. As an example of true lips may be mentioned a member 

 of the genus Ascaris (Fig. 42) as showing, when viewed en face, three 

 lobe-like projections of which a large one, bearing two papillae, is dorsal, 

 while the other two each bearing a single papilla, are ventral. The genus 

 Camallamus (Fig. 33>) illustrates true jaws. Here the armature is divided 

 into dextral and sinistral halves which act as a vise for maintaining a hold 

 on the host tissue. The capsule or third type may be found among the 

 members of the group of Strongyles (Fig. 40). Here the vestibule is cup- 

 shaped with a large roomy interior opening to the exterior by an oval or 

 circular orifice. Within the vestibule and on the walls are various organs 

 for cutting, piercing, gnawing, etc. A closer study of the cephalic region, 

 its structure and symmetry may indicate the value of these suggested 

 groupings or may indicate further groupings or means of relating the 

 nematodes within any one category, or still further may serve to show 

 which structure is the most evolved and of the highest type thus indicating 

 the evolutionary status of the species or genus among the members of its 

 genus or family respectively. 



In order to obtain the proper perspective of the value of the cephalic 

 structure either from a taxonomic or an evolutionary point of view, it is first 

 necessary to consider what may be termed the primitive nematode, and 

 then in this light determine whether cephalic organization has kept pace 

 with or lagged behind the specialization of the nematode body as a whole, 

 through which it is fitted to its environment. It is for this reason that 

 the primitive nematode is considered in regard to its form and symmetry 

 before the anterior portions of the free living and parasitic forms are 

 discussed critically with reference to their symmetry and specialization in 

 structure. 



f. The great difficulty in such a definition or delineation arises in the 

 determination of criteria for primitiveness. In general a primitive organism 

 is believed to be one with the most generalized structure or in other words 



