36 ILLINOIS BIOLOGICAL MONOGRAPHS 1140 



This condition is still more apparent in Kalicephalus inernus Molin (Fig. 20) 

 where the capsule is rather more compressed and the appearance of jaws 

 accentuated. The supporting ribs are united anteriorly but are separated 

 posteriad. The exact genesis and significance of this disymmetry and of 

 the jaws in Camallanus cannot be known until the larval developments 

 have been carefully examined, but from adult features their similarity 

 cannot be structurally the same. 



Turning now to a consideration of parasitic forms which possess cephalic 

 structures arising from a variation of numbers of lips particularly other 

 than three, one finds interesting indications of fusion and capsule formation. 

 The lack of larval and embryological studies on most nematodes makes the 

 problem rather difficult, but as this evidence is unobtainable, conclusions 

 must be based on adult structures. However, as a matter of fact, Seurat 

 has shown in his studies on larval forms that many features of the young 

 do not differ strikingly from those of the adult. The general cephalic 

 structure is often the same, giving little information regarding its evolu- 

 tion. The reproductive systems, however, have been chosen as showing the 

 most phylogenetic facts. Furthermore, in larval comparisons care must be 

 taken to distinguish between structures of apparent phylogenetic impor- 

 tance and those cenogenetic in nature. 



Among the oxyurids there are three lipped forms (0. obvelata), six lipped 

 ones (0. hydroi Caleb), and some with a capsule apparently arising from a 

 fusion of lips (0. obesa Diesing and 0. equi Schrank), and there is even a 

 two-lipped species (0. monhystera von Linstow). It is open to question, 

 however, whether the two lipped and capsulated forms are true oxyurids 

 and whether such a diversity of form might not better indicate a generic 

 rather than a specific relationship. 



In the family Filaridae the members are characterized partially by their 

 lack of cephalic armature. No pharynx is present; the head is usually 

 rounded with circular mouth opening and with few exceptions no lips are 

 present although cephalic papillae may be apparent existing in the numbers 

 of four and six. Such details would naturally be expected among forms 

 reduced to such a degree of parasitism as these nematodes are. Filaria 

 bancrofli Cobbold, for example, has an unarmed circular mouth and two 

 circlets of six low inconspicuous cephalic papillae while F. candezei Seurat is 

 similarly constructed except that the second circlet of papillae contains 

 only four. AcanlhocJieilonema diacantha from the body cavity and lungs 

 of various Brazilian rodents possesses a circlet of six papillae of which the 

 median lateral ones are very large, giving the head a square outline in 

 dorsal or ventral view. A curious feature of the head region of members 

 of the filarid genus Diplotriaena {D. diuca Boulenger) is the occurrence of 

 two lateral chitinous tridents reminiscent of those in Camallanus except 

 that in this latter genus they are dorsal and ventral tridents in connection 

 with the lateral valves. 



