141] COMPARATIVE STUDIES ON NEMATODES— HETHERINGTON 37 



A slight modification of the filarid head suggesting the possibility of 

 two lateral lips is found in Selaria equina Abildgaard, where the mouth 

 is surrounded by a chitinous ring, the lateral portions of which are pro- 

 jected as two semilunar lips. There is as well on the dorsal as the ventral 

 surface a papilliform process and at a lower level on the head there are four 

 submedian prominent papillae. In all the filarid worms just mentioned, 

 with perhaps the exception of the first, the symmetry is bilateral, chiefly 

 so on account of the hypertrophy of the lateral papillae or because of the 

 presence of four submedian in place of six radially placed papillae. 



A very noticeable case of radial cephalic symmetry exists in Sp^irocera 

 subaequalis Molin (Figs. 24 and 32) in the adult form. The six denticles 

 of the two lateral lips are regularly disposed around one axis and the 

 buccal border is cut into sLx equal lobes. This radial symmetry is, however, 

 secondary because in the larval stages of this particular worm, the sym- 

 metry is bilateral till the nematode has passed into the fourth larval stage. 



In the genera Eustrongylides and Hystrichis, the mouth is usually a 

 triangular or circular opening leading into a very short vestibule similarly 

 shaped in cross section. The buccal aperture is surrounded by six papillae 

 on very prominent projections which have a slight tendency to bilateral 

 arrangement although the radial appearance is more striking. Hystrichis 

 acantbocephalicus Molin illustrates this characteristic quite well (Fig. 29). 

 Species of Eustrongylides possess in addition to the six large papillae six 

 to twelve smaller ones, as E. elegans von Olfers (Fig. 36). 



Finally in the strongylids one finds the most interesting of oral develop- 

 ments in the form of a large armoured buccal cavity or pharynx. The oral 

 aperture is directed often dorsally as in the hookworms or terminally as 

 in the sclerostomes, but of these positions the dorsal location is purely 

 a secondarily acquired one, occurring late in the larval life. Ancylostoma 

 duodenale Dubini illustrates very admirably the general plan of such 

 cephalic parts. The ventral margin of the mouth which projects farthest 

 forward carries on each side of the middle line a pair of strong teeth with 

 backward bent prongs. The outer one is always larger and the inner one 

 has near its base on the side turned toward the median plane of the body a 

 small accessory tooth. The dorsal edge of the capsule shows in the middle 

 line a short and rather deep incision of which only the two anterior angles 

 project above the rim, because the greater part of this structure is covered 

 by the cuticula. Upon the ventral wall of the mouth capsule near the 

 base of the cavity is another pair of saw-like teeth projecting freely into 

 the cavity and converging backward only slightly. The dorsal wall of the 

 cavity is pierced obliquely from without inward by the excretory duct of 

 the dorsal esophageal gland. 



The whole mouth capsule is one continuous chitinous piece of material 

 which, however, can be changed slightly in shape through the presence in 

 its walls of several sutures where the hard parts are so thin as to permit a 



