DERIVATION OF PARASITES 21 



hath 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 rediae, Archigetes with cystic^rcoids. He classes 

 the nematodes that become mature in the invertebrates with Anguil- 

 Inlidce, i.e., with saprophagous nematodes from which the parasitic 

 species descend. 



Leuckart therefore regards the change of hosts 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 con- 

 vinced that the migrations of the helminthes, as well as the system 

 of intermediate hosts, represent the original order of things. Moniez 

 traces all Entozoa from saprophytes, but only a few of these were 

 able to settle directly in the intestine and there continue their 

 development. These are forms that at the present day still lack 

 an intermediate host, such as Trichocephalus, Ascaris, and Oxyuris. 

 In most other cases the embryos, however, consisted of such sapro- 

 phytes 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 leave the 

 intestine at once, and accomplished this by penetrating the intestinal 

 walls and burrowing in the tissues of their carriers. In this position, 

 assisted by the favourable conditions of nutrition, they could attain 

 a relatively high degree of development. Mechanical reasons pre- 

 vented a return to the intestines, where the eggs could be deposited. 

 Most of them doubtless died off as parasites, as also their young 

 stages do at present when they penetrate wrong hosts. Some of 

 them, nevertheless, passively reached the intestine of beasts of prey. 

 Many were destroyed in the process of mastication ; for a small part, 

 however, there was the chance of reaching the intestine of a beast 



