An explanation of the CHANGE OF HOST so frequent in parasites is more difficult than that of their descent. R. Leuckart is of opinion that the present intermediate hosts, which belong principally to the lower animals, were the original hosts of the parasites, and fostered both their larval and adult stages. It was only in course of time that the original hosts sank to the position of intermediate hosts, the cause for this alteration being that the development of parasites, especially of the helminthes, through further development and differentiation extended over a larger number of stages. The earlier stages remained in their original hosts, but the later stages sought out other hosts (higher animals). To prove this, Leuckart points out that the mature stages of the helminthes, with but few exceptions, occur only in the vertebrates which appeared later in the development of the animal kingdom, while the great majority of intestinal worms of the lower animals only represent young stages, which require transmission into a vertebrate animal before they can become mature. The few helminthes that attain maturity in the lower animals (_Aspidogaster_, _Archigetes_) are therefore regarded by Leuckart as primitive forms, and he compares them with the developmental stages of helminthes, _Aspidogaster_ with rediæ, _Archigetes_ with cysticercoids. He classes the nematodes that become mature in the invertebrates with _Anguillulidæ_, _i.e._, with saprophagous nematodes from which the parasitic species descend.