FERTILITY OF PARASITES 5 



early stage, the ordinary characteristics of other threadworms. A 

 few weeks later the males having died off immediately after copu- 

 lation the females are transformed into spindle-shaped bodies, the 

 mouth and anus of which are closed. They carry with them an irregu- 

 larly shaped appendage, in which the segmenting ova are situated, 

 and in which the further conditions of life of the Atractonema are 

 accomplished. A minute examination has demonstrated that this 

 appendage is the prolapsed and enlarged vagina of the animal which 

 has become merely a supplementary attachment. The conditions 

 present in the Sphcendaria, the nematoid nature of which was long 

 undiscovered, are still more remarkable. It was only when Siebold 

 proved that typical nematodes were hatched from their eggs 

 that their nature was recognized. The nematodes thus produced 

 have not the slightest resemblance to the parent. 



The researches of Lubbock, A. Schneider, and more particularly 

 of R. Leuckart, have shown that what we call Sphcendaria bombi is not 

 an animal but merely an organ the vagina 3f a nematode worm. 

 This vagina at first grows, sac-like, from the body of the tiny nema- 

 tode ; it gradually assumes enormous dimensions (2 cm. in length) ; 

 it contains the sexual organs and parts of the intestine. The remain- 

 ing portion of the actual animal then becomes small and shrivelled ; 

 it may be easily overlooked, being but an appendage to the vagina 

 with its independent existence, and it finally disappears altogether. 



The GREAT FERTILITY of parasites is another of their peculiarities, 

 though this may be also the case to a certain degree with some of 

 the free-living animals, the progeny of which are likewise exposed 

 to enormous destruction. 



More remarkable, however, is the fact that the young of the 

 endoparasites only very exceptionally grow to maturity by the side 

 of their parents. Sooner or later they leave the organ inhabited by 

 the parents, frequently reach the open, and after a shorter or longer 

 period of free existence seek new hosts. During their free period, 

 moreover, a considerable growth may be attained, or metamorphosis 

 may take place, or even multiplication. In the exceptional cases in 

 which the young remain within the same host, they nevertheless 

 usually quit the organ inhabited by the parents. They likewise rarely 

 attain maturity within the host inhabited by the parents, but only, 

 as in other cases, after having gained access to fresh hosts. 



These transmigrations play a very important rdle in the natural 

 history of the internal parasites, but they frequently conceal the cycle 

 of development, for sometimes there are INTERMEDIATE GENERATIONS, 

 which themselves invade intermediate hosts. Even when there are no 

 intermediate generations, THE SYSTEM OF INTERMEDIATE HOSTS is 

 frequently maintained by the endoparasites. 



