The structure and ultrastructure of the excretory system of Panagrellus redivivus, Ditylenchus myceliophagus, D. dipsaci and the second-stage larvae of the Heterodera rostochiensis are described. Panagrellus has two lateral ducts, a median duct and a gland. The two lateral ducts are joined over the anterior part of the oesophageal bulb to form a loop. Anteriorly this loop continues as a median duct, which opens to the exterior in an excretory pit, part of the system lies free in the pseudocoelom. The lumen of the lateral excretory duct and the greater part of the median duct is surrounded by capitate tubules, which open into the lumen of the excretory duct. Capitate tubules develop by the invagination of the hypodermal membranes and lie embedded in thick cytoplasm surrounded by mitochondria. Ensheathed in cell membranes the excretory duct forms an anastomosis in the loop and at three other places along the length of each lateral excretory duct. At these regions there are nerve endings. The excretory gland opens posteriorly into the loop and is full of Golgi complexes, endoplasmic reticulum and secretory granules.
In the system of Ditylenchus and the second-stage larva of Heterodera the right lateral duct is absent while the left lateral excretory duct continues anteriorly past the median excretory duct; a true loop is not formed. The median excretory duct lies embedded almost throughout its length in the excretory gland. Capitate tubules are present along the length of the lateral excretory duct, but, unlike Panagrellus, are not present around the median excretory duct. Anastomosis is not formed in the excretory ducts. In other respects the system of the species studied is similar to that of Panagrellus.
The capitate tubules form a system of fine microtubules. Their ultrastructure and, particularly, their association with large numbers of mitochondria, is very reminiscent of structures thought to be concerned with an active transport role, related to ionic and osmoregulation in other phyla.
The excretory system is almost certainly hypodermal in origin. Differences between the morphology and relations of the median duct of Enoplus (Narang, 1970), on the one hand, and that of the other forms studied suggest that these ducts may have different origins: the presence of separate nuclei in the lateral duct and gland portions of the system would be consistent with this view. It is considered that the two parts may have originated separately and have become joined during evolution. It is also shown that the excretory system of Ditylenchus cannot be derived either from an H-shaped system or from a primitive gland system.
I wish to thank Professor C. Ellenby and Mr L. Smith for their helpful criticism.