466 FRESH-WATER BIOLOGY 



for instance, in the case of the supplementary organs of the males. 

 These papillae, hairs, and setae all belong to the same general class of 

 structures, but various terms are applied to them in accordance with 

 their size and location. The special hairs found on and near the 

 lips are known as cephalic setae, in contradistinction to the large 

 hairs or setae sometimes found at the posterior extremity, the 



caudal and terminal setae. The 

 setae are no doubt mainly tac- 

 tile in function, though it seems 

 certain that some of the ce- 

 phalic setae and papillae serve 

 also as organs of taste and 



f FIG. 768. Head of a nematode (diagrammatic), i.side smell, 

 view; 2, front view, showing triangular mouth opening 



inthemiddle. The ventral side to the right in i shows The similar or^an^ fmiTlH on 



the ampulla and excretory duct. As the right side of the [IUld/r ur g<"> Ioun< 



head is towards the spectator the lateral organ appears th~ rrpnpral cnrfirA rf tVip KrH\r 



as a left-handed spiral. The arrangement of the cephalic me general SUHaCC OI U1C DOQy 



^^SSS^^&S 1 ^^^ are called hairs or somatic 



setae. These probably follow a 



definite law in their distribution, but are so small that the exact 

 distribution is difficult to make out and has been studied in but 

 few cases. While it is not established that their distribution 

 accords with a segmentation theory, this matter is worthy of care- 

 ful study. Sometimes the hairs occur in harmonic repetition on 

 successive groups of annules. The papillae of the cuticula are setae 

 that do not project beyond the surface, or not far enough to entitle 

 them to be called setae. They should not be confounded .with 

 pores, or with mere projections of the surface of the cuticula. 

 Neither of these latter are innervated. Tactile structures sup- 

 plementary to the sexual organs are found on the tail end of the 

 male both in front of and behind the anus, generally toward the 

 ventral side. They are much more rare in the female, being 

 located, when present, near the vulva. 



What are known as the amphids or lateral organs are of such 

 widespread occurrence among free-living nematodes as to make it 

 seem certain that their function is of fundamental importance, but 

 what the function is remains a mystery. The amphids are two 

 lateral, symmetrically-placed external cephalic organs. The ex- 

 terior part has the form of a circle, spiral, helix, or elongated figure, 



