22 THE ANIMAL PARASITES OF MAN 



opinion that the present intermediary hosts, which belong princi- 

 pally 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 

 intermediary 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 verte- 

 brates 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 (Aspido- 

 gaster, Archigetes) are therefore regarded by Leuckart as primitive 

 forms, and he compares them with the developmental stages 

 of helminthes, Aspidogaster going through a change of host with 

 Redia, Archigetes with Cystercoidea. He classes the nematodes 

 that become mature in the invertebrates with Angui&ulida, i.e., with 

 saprophagous nematodes from which the parasitic species descend. 



Leuckart therefore regards the change of host as secondary, 

 so does Sabatier. The latter, however, adduces other reasons 

 for this (lack of clinging organs and the necessity to develop 

 them in an intermediary stage) ; but in this connection he only 

 considers the Cestoda. In opposition to Leuckart, R. Moniez, 

 however, is convinced that the migrations of the helminthes, as 

 well as the system of intermediary hosts, represent the original 

 order of things. Moniez traces all Entozoa from saprophytes, but 

 only a few of these were able to settle directly into the intestine 

 and there continue their development. These are forms that at 

 the present day still lack an intermediary host, such as Tricho- 

 cephalus, Ascaris and Oxyuris. In most other cases the embryos, 

 however, consisted of such saprophytes as were, in other respects, 

 suitable to become parasites but were incapable of resisting the 

 mechanical and chemical influences of the intestinal contents ', 

 they were therefore obliged to at once leave the intestine, and 

 accomplished this by penetrating the intestinal walls and burrow- 

 ing in the tissues of their carriers. In this position, assisted by 

 the favourable conditions of nutrition, they could attain a rela- 

 tively high degree of development. Mechanical reasons pre~ 



