Variability of the Basal esophageal sheath in belondirid nematodes.

The structure of the basal esophageal sheath was studied in Axonchium amplicolle, A. choristurn, A. crassum, A. gigas, A. micans, A. rotundum, A. serpens, A. solitare, Dorylairnellus aequalis, D. parvulus, D. tenuidens, Oxydirus gangeticus, O. gigus, O. oxycephaloides, and Swangeria bisexualis. In A. gigas the muscle bands comprising the sheath spiral dextrally, but in the other species of Axonchiurn they do not spiral but lie parallel to the esophagus. In O. gigus the spiral is sinistral, whereas in the other two species of Oxydirus it is dextral. The spiral is very slight in Dorylairnellus aequalis, each band passing around only about 180 degrees of the circumference of the esophagus. They completely encircle the esophagus in D. parvulus, and pass around about 3/4 the circumference in D. tenuidens. The definitions of Belondiroidea Thorne, 1964 and of Axonchium Cobb, 1920 are emended to recognize the variability in the structure of the sheath.