The Life Cycles of Nematodes in the Family Ascarididae Blanchard 1896

Abstract

Evidence for utilization of intermediate hosts by members of Ascarididae is summarized; it is concluded that this is a widespread feature, occurring in both terrestrial and aquatic members of the 2 subfamilies Anisakinae and Ascari-dinae. Use of intermediate hosts is regarded as a primitive characteristic. The various spp. of animals at present known to be used for this purpose indicate the possibility that this family originated as parasites of marine arthropods, and that radiation has occurred among marine animals through the use of marine invertebrates and fish as intermediate hosts. The spread to terrestrial animals may have occurred through terrestrial co-prophagous animals, such as rodents, swallowing ascaridid eggs from the feces of littoral animals. By harboring encysted larvae in their tissues, rodents may have become intermediate hosts for the ascaridids of many terrestrial carnivorous animals. In extending their range to non-carnivorous hosts, the ascaridids have modified the migratory behavior of the larvae so as to dispense with intermediate hosts, and promote infection of the final hosts through the ingestion of the embryonated eggs. In some spp., e.g. Toxocara canis, the somatic type of migration has been retained and infection occurs through penetration of the larvae into the fetus; in other species, e.g. Ascaris suum, the somatic migration has been dispensed with in favor of tracheal migration; in Toxascaris leonina. migration has been limited to the abdominal region so that the larvae may return to the intestine directly.