134(3.3.436) THE MONONCHS 



known to have this faculty, which is due to repulsion between the cuticle 

 and water ; de Man found it to exist in the case of Oncholaimus viscosus, 

 and the writer has observed it in the case of the larvse of Diplogaster 

 aerivora, although a second larval form of this same species does not 

 exhibit the property, nor do the adults. Ability to float is known also of 

 a number of other species, and, as Ditlevsen's observations show, is not 

 absent in the genus Mononchus. Whether flotation plays any part in the 

 economy of such nemas is not known with certainty. Floating on the 

 surface of moving water, nemas would doubtless sometimes be quickly 

 and widely distributed, and it is therefore easily conceivable that flotation 

 has some definite bearing on the life history of species exhibiting it. 



The surface of a mononch seems almost 



*' Jfc ' >^^-^H -^T^ ' entirely destitute of setae and papillae, if 



we except those on the lips. Neverthe- 

 less, future researches will probably re- 

 veal superficial nerve-endings hitherto 

 overlooked. In rare instances a few fairly 

 well-developed setae occur on the tail, as 

 in the case of M. sigmaturus; papillae also 

 occur sparingly on the tails of the males 

 of various species, and near the vulva on 

 the females of a number of species. No 



the lips are nearly closed, but the dor- pores are known tO OCCUr in the CUticle, 



Si", Sr~'SS except, of course, the spinneret pore, and 



of papilla: are seen to good advantage, the pore observed near the nerve-ring and 



to be the renette pore. 



General Form of Body 



Neck and Head. The various mononchs are considerably alike in 

 external form. In front the body tapers but little; the neck is nearly 

 always more or less cylindroid, and almost invariably ends in a head not 

 set off from the neck in any way, though there is usually a slight expan- 

 sion at the lip-region, due to the strong development of the labial papillae. 



Tail and Spinneret. The posterior portion of the body usually tapers 

 from some distance in front of the anus, but in the pre-anal region the 

 diminution is slight and very gradual. The tail may be either simply 

 conoid, or first conoid and then cylindroid, and usually ends in a spin- 

 neret, though in about one-fourth of the species the caudal glands and 

 spinneret are absent. When the tail is conoid the spinneret is usually a 

 simple structure, whose existence is indicated mainly by the fact that the 

 terminus is sub-truncate in form, and presents an inconspicuous axial, 

 or sub-dorsal, or sub-ventral pore. This form of spinneret appears to be 

 entirely unarmed, though inconspicuous innervations probably occur. 



