48 LIFE-HISTORY OF PARASITES. 



It is, moreover, not without significance that all these cases of 

 "worm-nests" belong to the Nematodes. Seeing that it is an un- 

 doubted fact that there are parasites whose eggs remain, without further 

 development, in the same place where they were deposited, and are not 

 extruded from the body, as is the rule in other cases, the fate of the 

 embryos that arise from such eggs remains to be examined. The most 

 evident supposition is that these embryos grow to maturity in the 

 same spot by the side of their parent, and this is quite true of certain 

 parasites. It is well known, for instance, that the young lice grow to 

 maturity on the spot where they were born, and the investigations of 

 Wagner, Zeller, and myself have shown that this is also 

 the case with the above-mentioned gill-parasites, at 

 least Dactylogyrus, Diplozoon, &c. 



The life-history of such parasites thus becomes 

 extraordinarily simple. One generation follows another 

 without any change being necessary, either to another 

 host or another organ. If there be any migration, it is 

 due to a mere accident. 



So far as we know, it is only Epizoa which have 

 a simple life-history of this kind, though it has been 

 attempted to prove that certain entoparasites, especially 

 thread-worms, reach maturity without a change of 

 locality. This opinion has, however, been shown to 

 be incorrect, even in the case of the maw-worm (Oxyuris 

 vermicularis), which is generally found in vast quantities 

 in the human alimentary canal, and on that account 

 would seem most apt to support such a theory. * 



Neither has the statement of Norman been con- 

 firmed, according to which all the developmental stages 

 FIG. 30. Rhab- of Anguillula (Rlwibditis) stercoralis should be abun- 

 ditis terricolai dantly met with in the viscera of persons suffer- 

 ing from "Cochin-China diarrhoea," This worm is, as above 



in reality Psorosperms (Coccidium, Lt.), which used frequently to be mistaken for eggs 

 (see postea). Virchow described (Archivf. path. Anat., Bd. xviii., p. 523) a genuine case 

 from the liver ; the eggs, however, proved not to be Pentastomum, as Virchow thought, 

 but Ascaris lumbrieoides, from an examination that I made of some specimens that 

 were sent to me, which were previously forgotten. Thus there is one case of the 

 presence of a thread-worm in the bile duct (see Vol. II. ) So, too, with the " worm-nests " 

 of Trichosomum described by von Siebold in the spleen of a shrew-mouse (Archiv f. 

 Naturgesch., Jahrg. xiv., Bd. ii., p. 358, 1858). The bodies of the worm were found 

 twisted together in knots near the eggs. 



1 In support of this statement, which is at variance with the opinions of Kiichen- 

 meister (" Parasiten des Menschen," first ed., p. 229) and Vix ("Ueber Entozoen bei 

 Geisteskranken," Zeitschr. f. Psychiatric, Bd. xvii.), I may quote my own observations de- 

 scribed in Vol. II. of this work, which have also been confirmed by Zenker (Abhandl. 

 der physile. med. Societal zu Erlangen, Hft. 2, p. 20, 1872). 



