2 NATURE AND ORGANIZATION OF PARASITES. 



Under this limitation, the group of parasites appears at first sight 

 to be considerably more restricted than it did from the former 

 wider point of view, and in earlier days, when it was thought that 

 parasites always existed as parasites, for the simple reason that they 

 were unable to lead a free existence, even more restricted than now. 



Modern investigations have taught us that there are frequently 

 stages in the existence even of the most thorough-going parasites, such 

 as the intestinal worms, when they lead a free life in water or damp 

 earth ; and also that among the thread-worms there are many species 

 (e.g., Rhabditis) that are occasionally parasitic, and capable of arriving at 

 their full development in milk, meat, and other organic substances as 

 rapidly as, if not more quickly than, in the interior of a living organism. 

 In another thread-worm (Ascaris nigrovenosa, Auct.), we have an in- 

 stance of an animal whose life-history consists of two alternate gene- 

 rations, 1 both sexually mature, which differ so much from each other 

 in structure and mode of life, that, before their genetic connection 

 had been discovered, they were referred to two distinct families. 2 

 This case, which has such an important bearing upon the meaning 

 and right understanding of the phenomenon of parasitism, will be 

 described more fully in a subsequent chapter. 3 It follows, therefore, 

 from a case like this, that certain animals, such as the larvae of many 

 flies (Miisca vowiitoria, SarcopJiaga carnaria, Anthomyia canicularis, 

 &c.), which occasionally feed upon living animals, although usually 

 found in dead putrifying organic matter, are by no means to be ex- 

 cluded from the category of parasites. If this kind of parasitism is 

 to be distinguished in any way, it may conveniently be termed 

 " occasional," in contrast to the " constant " parasitism exhibited by 

 other animals. The term " pseudoparasite," which has been fre- 

 quently, even in recent times, applied to cases of this kind, ought to 

 be confined to various objects, such as hairs, vegetable tissue, &c., 

 which have been mistaken for parasites, and even described as such; 4 



1 For a fuller account see Vol. II. 



2 The case mentioned in the text is not the only one known. Recent investigations 

 have shown me that Anguittula stercoralis, occurring in cases of " Cochin-China diarrhoea," 

 is the Rhabditiform generation of A. intestinalis (Leuckart, Bericht math. phys. Cl. I: 

 Sachs. Gcsdlsch. d. Wiss., p. 85, 1882). There lives also in the peritoneal cavity of Jli/lnbinx 

 pint a strange parasite, Allantonema mirabile, Lkt., whose offspring leads a free existence, 

 and gives rise to many generations of Rhabditis, like sexually mature worms (Leuckart, 

 Tagebl. d. Magdeburg. Naturf. Versamml., p. 320, 1880. R. L. 



3 Chapter VIII. 



4 A list of this kind of pseudoparasites, including only the commonest, would be too 

 long for insertion here. It may be remarked that all kinds of objects, not only the debris 

 of food (orange-pips, raisin-stones, sinews, small bones, and so forth), but also pieces of 

 thread, hairs, &c. , have been mistaken for parasites. It is generally not difficult to dis- 

 tinguish these by the help of the microscope. 



