FREE-LIVING NEMATODES 463 



some of the markings usually to be seen in the cuticula are due to 

 sense organs or to pores, most of them are inherent structural 

 markings. These markings are used as specific, and in some cases 

 as generic, characters. 



The cuticula of almost any species, if examined with sufficient care, 

 will show transverse striations, ranging in the various species from 

 a few score to upwards of a thousand. 

 Many species described by earlier writers 

 as destitute of these striations really 

 possess them. When very fine the 

 transverse striae are best seen at the 

 extremities of the organism. In some 

 genera the striae are apparently due to 

 the constant bending of the body in the 

 dorso-ventral plane. This peculiar mo- FIG ?6? Diagnun atic cross . section 

 tion, which is universal among nema- S^%f f n ^ t l ; a 

 todes and continuous from birth to 



i .-, i . i i ,-, lateral fields. The median, lateral, 



death, Unceasingly Stretches and then and submedian lines are imaginary 



. , lines opposite the letters m, I, and sm, 



COmpreSSeS the dorsal and Ventral SUr- respectively, these lines being, of 



course, in no sense organs, but only 



faces. At the time When the One IS convenient descriptive terms. The 



line shading between the fields repre- 



stretched the other is compressed. This sents muscle ceUs - < After Cobb -> 

 results in characteristic appearances, such as the more pronounced 

 striation of the dorsal and ventral surfaces, the interruption and 

 variation of the striations near the lateral lines, and the presence on 

 the lateral fields of various longitudinal striations. In some genera 

 the striations are compound, that is, each transverse striation is 

 resolvable into a row of dot-like markings, either round or elongated. 

 These secondary markings may be again resolvable, the result being 

 a very complex series of exceedingly minute markings. The trans- 

 verse striations are usually more or less plainly interrupted near the 

 lateral lines. Oblique striae, such as are to be found in the large 

 parasitic nematodes, sometimes occur in the fresh-water species, 

 in some Mermithidae, for instance. 



The longitudinal striations are of two kinds: (i) True stria- 

 tions of the cuticula due to certain stiffening structures or "wings," 

 and (2) internal markings due to the attachment of the cells of the 

 muscular layer and of the lateral fields. The longitudinal stria- 



