Ultrastructure of the hemizonid.

The hemizonid (6) is a ventro-lateral, commissure (7) which extends around the ventral side of nematodes between the hypodermis and the cuticle, stopping at the lateral fields (2, 4, 6, 8). Although the hemizonid has been recorded mainly from plant parasitic nematodes (1,4, 6, 7, 8), Timm (11) reported its presence in a wide variety of nematodes and suggested that it may be present t h r o u g h o u t the Nematoda . Observations on the hemizonid have to date, been at a light microscope level. However, Bird (2, his Fig. 3) suggested that his electron micrographs of second-stage larvae of Meloidogynejavanica (Treub.) may show a structure which is part of the hemizonid. Bird (3) also suggested that the ventro-lateral commissure described by Rogers (10) in Haemonchus contortus (Rudolphi) and situated posteriorly to the excretory duct, may have been a hemizonid. Because of the uncertainty that clearly exists in interpreting observations made with the light microscope and the electron microscope, an attempt was made to characterize the fine structure of the hemizonid of the secondand third-stage larvae of the cat hookworm, Ancylostoma tubaeforme (Zeder) and third-stage larvae of the human hookworm, Necator americanus (Stiles). The worms were fixed using the technique previously described (5) for small nematodes, and longitudinal sections of the oesophageal region were cut on an LKB ultramicrotome using glass knives, mounted on Formvar coated grids, stained with uranyl acetate and lead citrate and examined with a Philips EM 300 electron microscope operated at 60 and 80 kV.