Toxocara canis: ultrastructural aspects of larval moulting in the maturing eggs.

The morphology of the surface of Toxocara canis larvae, developing in the eggs to reach infectivity, has been studied for the first time at an electron microscopical level. In most 11-day and some 15-day eggs, the larvae are surrounded by two shed cuticles. The outer first shed cuticle is composed of two layers. the inner second shed cuticle is much thicker than the outer one. The presence of both shed cuticles indicates that the larva has undergone two developmental stages in the maturing egg. The larvae in most 15-day eggs are surrounded by one shed cuticle composed of outer electron-dense and inner layers. This cuticular sheath is identical with the described inner second shed cuticle, except for its apparently reduced thickness. The infective larvae inside the 30-day eggs are enveloped by one cuticular sheath, derived from the second moulted cuticle, and consisting only of a single layer. The findings are discussed with respect to data concerning the moulting process in other nematode species.