Differentiation of cuticular structures during the growth of the third-stage larva of Ascaris suum (Nematoda, Ascaridoidea) after emerging from the egg

In order to monitor the early phases of the development of Ascaris suum fro m domestic pigs, third-stage larvae, retrieved from the liver and the lungs , were studied by analyzing worm growth and length increase of individual t ransverse annuli in the cuticle. Material for study using light and scannin g electron microscopy was obtained from experimental infections. The result s show that the third-stage larva (not the second-stage) after emergence fr om the egg grows continuously, without an ecdysis in the liver. During grow th, each annulus is split into a complex of 2 subannuli, each of which atta ins a bimodal appearance and is a prominent feature during a late phase of the third-stage larva. The results suggest that the first 2 molts occur ins ide the egg, a synapomorphic feature of the Ascaridoidea. The third-stage l arvae of ascaridoids, with some functional similarities of the dauer-larva stage of Caenorhabditis sp., facilitate transmission of these parasitic wor ms to the digestive tract of the vertebrate final host (utilizing the trach eal route in A. suum), where the third and the fourth molts take place.