Females. Lip region not offset. Spermathecae weakly developed or absent. All other described species of Belonolaimus with lip region offset and spermathecae generally well developed.
Males. Stylet, stylet knobs, median esophageal bulb and esophagus rudimentary or poorly developed, gubernaculum with anterior flexure 2.9 microns (1.4-4.2) long.
Immature Stages. B. euthychilus is separated from B. longicaudatus in the larval stages emerging from the eggs by the absence of a constriction in the neck region and the central lateral line extending from the tip of the head to the end of the tail, while the other two lateral lines begin 12 to 18 annules from the neck and extend to the end of the tail. In larval stages of B. longicaudatus there is a constriction in the neck region and the central lateral line is present in the head, absent in the first 8 to 10 annules from the head and begins again extending to the end of the tail; the other two lateral lines begin 40 to 50 annules from the neck constriction.
B. euthychilus is easily distinguished from B. gracilis by the qualitative characters but appears to be generally similar to it in measurements of total, anterior, posterior, stylet and tail lengths. However, analysis of variance of these characters indicate the differences are significant at the 95% level.