139] COMPARATIVE STUDIES ON NEMATODES— BETBERINGTON 35 



or beset with chitinous rose-thorn hooks with their roots buried well in the 

 cuticula; for example Tanqua tiara von Linstow (Fig. 25) has five notched 

 lips and coarsely striated cuticula, while Gnathostoma spingerum Owen 

 (Fig. 34) has less elaborate lips and a spine beset collar. These forms 

 live with few exceptions within the digestive tract of various animals. 

 Their symmetry is undeniably bilateral as is that of the forms cited in the 

 previous paragraph, and seems to have arisen by loss of the dorsal cephalic 

 sector. In view of the fact that they also show a considerable specializa- 

 tion in the reproductive organs as well as in the cephalic region, one may 

 say that this secondary return to fundamental bilaterality from the 

 apparent radial symmetry of the three lipped forms agrees with the same 

 condition found among the free living nematodes, as has already been 

 demonstrated. 



In connection with bilateral disymmetry there are quite a number 

 of forms which would fall into the class of individuals with trisymmetrical 

 cephalic regions if it were not for the development of two very pronounced 

 median lateral anterior alae or the distinctly bilateral arrangement of 

 papillae (Spiroptera papulosa Molin, 5. turdi Molin and Oxyiiris obesa 

 Diesing, for papillae and O. tetraptera von Linstow, for alae). The Camal- 

 lanidae are conspicuous for their lateral jaws. These consist of two valves 

 similar in shape to Pecten or scallop shells, brownish in color and free only 

 along the dorsal and ventral edges of the anterior halves. Throughout 

 the other half they are fused so that cross sections exhibit a more or less 

 oval chitinous ring. The interior surfaces are ridged and the exterior is 

 covered by a delicate layer of cuticula. Another distinguishing feature of 

 these nematodes is the pair of chitinous tridents, one dorsal and one 

 ventral, articulating with the valves. This type of oral structure is 

 apparently derived from a lip-like structure according to Magath, and 

 Raillet and Henry (1915) have placed the Camallanidae under Spiru- 

 roidea, a group which is characterized by lateral lips. As the lateral 

 disymmetry has in other spirurids arisen from loss of the dorsal cephalic 

 sector, the same loss may be responsible for the formation of jaws in 

 Camallanus, as is clearly the case among the free living nematodes already 

 referred to as possessing bilateral jaws or mandibles. 



Another oral organization which very closely resembles jaws in appear- 

 ance and apparent function also is to be found in the genus Kalicephalus 

 and perhaps less distinctly in Diaphanocephalus, both bursate nematodes 

 of unsettled classification as yet, but according to Stossich apparently be- 

 longing to the Sclerostomes. The mouth capsule of Diaphanocephalus 

 costatus Diesing is transparent and armed or strengthened by eight cuticular 

 or chitinous ribs running vertically and between these buried deep in the 

 capsule are six papillae. The striking feature of the capsule is that the 

 buccal orifice in place of being circular is a spindle-shaped opening as if the 

 edges of the capsule had been compressed and approximated laterally. 



