96 THE ORIGIN OF PARASITES. 



sexual maturity only after abandoning this host, when they cast 

 their skin, and lose their riband-shaped caudal appendages, while the 

 apertures of the alimentary and sexual organs break outwards 

 through the cuticle. In the sexually mature state also the size and 

 formation of the tail characterise these animals as a peculiar form. 

 Even the internal organization shows many differences. The uterus 

 contains at least 500 to 600 eggs, whilst in the female developed from 

 the free larva it encloses two or three dozen eggs at the utmost. In 

 both cases, however, the eggs develop within the body of the female into 

 embryos, which are exactly alike in size, form, and organization ; and 

 may also attain to sexual maturity in the free state in the presence of 

 nitrogenous food material, without the need of migration into slugs. 

 Hence there is no doubt that the parasitism in this case is merely 

 collateral with the free state, and is of importance in the maintenance 

 of the species only so far as in agreement with the relations pre- 

 viously indicated it affords the possibility of producing a more 

 numerous progeny. At the same time it is evident that the devia- 

 tions in the structure of the parasitic generation are in correspon- 

 dence with the altered circumstances of its life, and are conditioned 

 by them. 



The appearance of parasitic generations side by side with free- 

 living ones, which in the case of the above-mentioned Rhabditis 

 appendiculata was only possible under certain circumstances, is 

 more conspicuous in other instances, and becomes ultimately a 

 constant phenomenon. The parasitic generations intercalate them- 

 selves between the free-living, in regularly alternating succession, 

 just as do the so-called " nurses " between the sexual animals in the 

 case of alternation of generations. But the intermediate generations 

 are not asexual like the nurses, which, as is well known, produce their 

 successors asexually, but they are complete sexual animals, equivalent 

 morphologically to the free-living generations, and in some respects 

 even occupying a position superior to them. 1 



Such is the case with the above-mentioned Rhabdonema (Ascaris) 

 nigrovenosum (p. 2), whose Rhabditis-foTm, living in the excrement of 

 frogs, differs very little from the animals related to it. Like other 

 species of Rhabditis of small size (Fig. 61), it attains sexual maturity 

 within a short time, and produces several embryos, which are hatched 

 within the body of the female, and, as has also been observed in the 

 case of other Ehabditida?, remain there until they have completely 

 destroyed and devoured the internal organs. Also, at the com- 



1 I have for some time been accustomed to call such an alternate succession of 

 dimorphous sexual generations by the name " Heterogeny." 



