135] COMPARATIVE STUDIES ON NEMATODES— BETHERINGTON 



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Cephalic Structure in Parasitic Nematodes 

 Symmetrical type of the esophagus 

 What has previously been said regarding the symmetrical factor of the 

 esophagus in free-living nematodes is true of the parasitic forms also, at 

 least those possessing the characteristic triquetrous muscular esophagus. 

 Those nematodes having a capillary esophagus referred to as belonging to 

 the group Trichosyringata comprise a restricted number of genera, among 

 them being Trichinella, Trichuris, Trichosomoides, Capillaria, etc. Species 

 belonging to these genera have no free-living larval forms and depend for 

 distribution not upon any migratory effort on their own part but rather 

 upon direct transmission of the embryos or eggs into the new host. 

 Trichina, as is well known, is spread by the ingestion of the uncooked 



I 



Text Fig. E. Diagram of a nematode head en face showing asymmetry. 



Text Fig. F. Diagram showing a typical case of bilaterality in a capsulated cephalic region. 



flesh of the host containing the encysted immature worms. The gravid 

 female of Eepaticola hepatica Hall dies in the liver tissue of the host, 

 leaving there a mass of eggs which have no way of reaching the exterior or 

 attaining a new host. In such a case, cannibalism seems to be the only 

 agent upon which the species can depend for propagation. From the point 

 of view of such extreme parasitism in this respect and further from the 

 loss of one or both spicules of the male, the absence of one testis and one 

 ovary in the respective sexes and the oviparous or ovoviviparous condition 

 of the female, one might be justified in suspecting that the capillary 

 esophagus is a degenerating one, departing in this respect from the normal 

 form. With regard to the symmetry, this type is bilateral, if the row of 

 large nucleated cells of the structure is dorsal and the capillary tube re- 

 mains in a ventral median position, or if the tube being intra-cellular, lies 

 within the dorso-ventral plane. 



Cephalic modifications and relations to habitat 

 The transition from a free to a parasitic mode of life brings with it 

 profound modifications in the organism. These changes are most admir- 



