121] COMPARATIVE STUDIES ON NEMATODES— HETHERINGTON 17 



lateral surface so that the actual creeping surface is in no way comparable 

 for example to the creeping surface of the earthworm. The embryological 

 evidence indicates that the actual adult and embryonic ventral surfaces 

 are the same so that there can not have been any shifting of the anal, 

 excretory, and reproductive openings to a lateral field. This fact proves 

 that the mode of locomotion engaged in by most nematodes has been 

 acquired as a secondary means of progression. 



The primitive orientation was probably of such a nature that the 

 principal axis was perpendicular to the substratum— the nematode being 

 held in position by the secretions of the caudal cement glands. In this 

 position the nematode could wave back and forth in a dorso-ventral 

 plane simulating the waving movements of some of the tubificid worms. 

 In support of this orientation are examples of some half sessile free-living 

 nematodes (from fresh and salt water) which live on algae and aquatic 

 vegetation and which may or may not possess eye spots. These sense 

 organs are blackish or red pigment spots or pigment cups, each of the 

 latter bearing over it a single transparent lens, as for example in the 

 following worms: 



Tlwracostoma antarcticum von Linstow 



Thoracostoma lobatum Steiner 



Nemella ocellata Cobb 



lonema ocellatmn Cobb 



Onchulella ocellata Cobb (Figs. 21 and 30). 

 These ocelli with few exceptions are to be found far forward lying laterally 

 upon the esophagus; only in a few cases do they lie slightly dorsal or ven- 

 tral with respect to the esophagus. The lenses are so directed that light 

 coming from a vertical source will fall upon them when the worm is ori- 

 ented vertically. Should the primitive worm have moved normally on a 

 side, as many of them do today, one or other of the eye spots would have 

 been turned toward the substrate and would have thus become temporarily 

 useless. Light to have stimulated both spots through the medium of the 

 lenses with the worm so oriented would necessarily have come from a 

 horizontal source which is improbable. Other nematodes, members of the 

 genus Echihdium, possess pigment surrounding the esophagus and above 

 this a circlet of ocelli set to collect vertically falling light. 



Furthermore some free living nematodes possess many long delicate 

 cephalic bristles, {Monhystera pilosa Cobb (Fig. 13), Pomponema mirabile 

 Cobb) and others bear on their bodies many fine bristles {Sphaerolaimus 

 hirsutus Bastian, Nolochaetosoma tenax Irwin-Smith). These delicate 

 processes can not be reconciled with a creeping mode of locomotion through 

 sand, mud, and debris, but they are consistent with a half sessile form of 

 existence. Still other species are parasitized by epizoa and epiphytes which 

 cover all portions of the body. For instance, a delicate filiform alga often 



