DURATION OF LIFE AND ULTIMATE FATE. 87 



The causes of these diversities are not yet known to us. We can- 

 not even explain why the liver or the intermuscular connective-tissue 

 has a different influence upon the development of the brood from that 

 of the brain. The supposition that the cause lies in certain differ- 

 ences in the supply and quality of the nutrition cannot of course 

 satisfy us. 



Here and there, however, the space afforded by the different organs, 

 and the peculiarities in the anatomical arrangement of the tissues, 

 may influence the development of the parasites. In this way the 

 bladder- worm in the brain of man grows in the subarachnoid space 

 into sinuous strings, dilated here and there into vesicles, which may 

 reach a length of 25 cm., but only rarely develop a head, so that the 

 true nature of this formation (the above-mentioned Cysticercus race- 

 mosus) has only recently been recognised. Also the so-called " multi- 

 locular " Echinococcus is perhaps only due to the position it occupies in 

 the body; whilst, on the other hand, the sterile Echinococcus (Acepha- 

 locyst) and the hydatid two other forms which can scarcely be con- 

 sidered as normal point to conditions of development which have to 

 be sought in another direction. 



There are only few animals in which the metamorphoses and their 

 individual phases are dependent upon the influence of external factors 

 to such a degree and in such an evident manner as is the case with the 

 Entozoa. The young parasite, if the opportunity of migrating to its 

 definitive host be wanting, remains stationary for life at a stage of its 

 development, which its more fortunate associates, and perhaps even 

 their descendants, have long ago left behind them. 



Moreover, it is not only the periods of migration and development 

 which cause Helminths to be dependent on external circumstances to 

 such an unusual degree. In their later period of existence also their in- 

 dependence is only a limited one. All the different disturbing influences 

 which attack the host, and endanger its health and existence, have 

 more or less an indirect influence upon the indwelling parasite. Some 

 may by certain steps be expelled from the intestine and other internal 

 organs, 1 whilst others may perish through the inflammatory condition 

 of their dwelling. For this reason also it is difficult to determine with 

 certainty the natural length of life of parasites. Concerning some, of 

 course, we know that they continue to live in exposed situations, not- 

 only for several years, but even through a whole decade (BothriocepJialus 

 latus, Tcenia saginata) ; but others again scarcely live longer than a 

 few weeks. On the whole, the period of existence of Entozoa may be 

 assumed to be longer than that of free-living animals of a similar size. 



1 Thread -worms often forsake the intestine on the occurrence of diarrhoaa. 



