131] COMPARATIVE STUDIES ON NEMATODES— BETBERINGTON 27 



they offer many interesting variations, finally becoming confluent and 

 forming a mouth capsule generally in connection with a large cavernous 

 pharynx or buccal cavity. Among the Mononchs as previously mentioned, 

 there are six lips which in many species are very well defined and separated 

 (Mononchus regius Cobb) and in others the six lips exhibit various stages 

 of union, for example, in Mononchus major Cobb the lips have become more 

 rounded and less distinct so that the mouth opening assumes a hexagonal 

 outline; in another genus and species, Bolbella tenuidens Cobb, the mouth 

 has become a perfect circle and the six lips have lost entirely their indi- 

 viduality externally, but internally the organization still indicates the 

 individual lips. An interesting feature, too, of this species is its asymmetry; 

 the amphids, instead of being mid-lateral, have shifted slightly and occupy 

 a dorso-lateral position and contrary to the general rule the dorsal onchus 

 is not the one which has become specialized but instead the sub-medial 

 dextral onchus has elongated and assumed a spear-like nature. 



A characteristic feature of the genus Anguillula is the entire lack of 

 lips; however, the papillae and internal arrangement of cuticular structures 

 indicate very distinctly that the capsule is the result of completely fused 

 lips, six in number (Fig. 5). In longitudinal section the mouth cavity is 

 definitely divided into two parts; first an upper vestibule, thin walled, with 

 the concave surface facing inward. This portion has probably arisen from 

 the under surface of the fused lips. These organs in many lipped forms 

 have a tendency to become thinner and less distinct so that one might 

 easily expect them to become still less thickened after fusing and losing 

 their identity to a greater or lesser extent. Following this vestibule (in 

 Anguillula aceti Miiller) there arises the pharynx properly speaking, set 

 off from the preceding structure by a distinct break in the cuticular wall 

 of the buccal cavity (Fig. 14). The lining of the pharynx is much thicker 

 and, as seen in cross sections of the pharyngeal region, the lumen of the 

 canal is triangular. The walls of the canal exhibit cuticular thickenings, 

 one in the mid-line of each sub-ventral sector, which are opposed to a small 

 triangular tooth in the mid-line of the dorsal sector. 



A similar distinction between pharynx and vestibule may be seen in 

 Monhystera stenosoma. Here the capsule has arisen from the fusion of 

 three lips. Cephalic papillae are not definitely known to exist but the 

 head bears in addition to the two lateral amphids four pairs of submedian 

 bristles, the anterior-most member of each pair appearing slightly shorter 

 than the other. 



Turning now to a consideration of the oral organization among the 

 spear-bearing nematodes, one finds lips again having undergone fusion 

 either partial or complete. Complete fusion occurs in the genus Tylen- 

 cholaimus, but evidence possibly of labial structure still remains in the 

 presence of six papillae surrounding the mouth. Other genera, Tylenchus 



