FREE-LIVING NEMATODES 465 



appearances. If the pharynx is armed with teeth these are often 

 seen in duplicate. In a moulting Dorylaimus, for instance, one may 

 see the old spear or tooth and behind it a second smaller one, and 

 in some cases even a third. The nature, or the presence, of stria- 

 tions may become more evident, or less evident, at the moulting 

 period than at other times. Remnants of old skin attached to 

 newly moulted individuals have sometimes given rise to erroneous 

 deductions and to errors in taxonomy. 



The nervous system centers in the so-called nerve-ring, which in 

 free-living species encircles the esophagus near the middle of the 

 neck. This ring is composed of interwoven nerve-fibers which, 

 taken together with the groups of nerve cells immediately in front 

 of and behind them, form what is undoubtedly a rudimentary 

 brain. (See Rhabdolaimus^) 



Eyes, or rather eye-spots, are known in one or more species of the 

 following fresh-water genera: Dorylaimus, Diplogaster, Spilophora, 

 Cyatholaimus, Chromadora, and Monhystera. The visual organs 

 in their most complete form consist of well-defined subspherical 

 cuticular lenses placed in front of collections of reddish, violet, or 

 blackish pigment-granules. Usually two such organs are placed 

 symmetrically, one on either side of the esophagus, between it and 

 the body wall, and in a dorsally sublateral position. Nerves pass 

 backward from the eyes to the nerve-ring. It is doubtful whether 

 the lenses form images that are perceived, though no doubt the 

 more perfect of the lenses found in nematodes are capable of form- 

 ing excellent images. Probably the lenses serve merely to collect 

 and condense light. Usually the eye-spots are mere collections of 

 pigment without lenses. Eye-spots, or what appear to be such, may 

 occur embedded in the esophagus. It is probable that the great 

 majority of species, even when without specialized visual organs, 

 perceive light by its direct action on the nervous system. A 

 few experiments will convince anyone that many eyeless species 

 distinguish the direction from which the light comes. There is no 

 satisfactory evidence that nematodes hear. 



At various points on the surface of the cuticula there are found 

 innervated papillae and setae, which appear in most cases to be tactile 

 organs. Sometimes, however, they are associated with glands, as, 



