68 



LIFE-HISTORY OF PARASITES. 



form passages of communication from one part of the body to another, 

 of which the embryos avail themselves. 



Whether these wanderings take place through the blood-channels, 

 through the connective tissue, or perhaps also directly through the 

 tissues of the organs themselves, and whether they commence at one 

 point or another, at the skin or the alimentary canal, one fact is certain, 

 that they do not last long. Sooner or later the embryo loses its activity, 

 and then, if the circumstances be favourable, undergoes, by grwvth and 

 metamorphosis, further development. 



These favourable conditions occur, perhaps, in only one definite 

 organ or host in a mammal, perhaps, or a snail, in the brain or in 

 the liver. Here only is a further development possible. If, as is 

 frequent, chance has brought it about that the young parasite finds 

 its way into some other animal or some other organ, it shortly dies ; 

 but in many cases it leaves behind traces of its presence. For in- 

 stance, in lambs that have been fed with embryos of Tcenia ccenurus, 

 which only attain to development in the brain, many other organs and 

 tissues, such as the muscles, connective tissue, and liver, are found to 

 be filled with minute cysts, which were no doubt at one time occupied 

 by the worms. 



The nature of the further development, of course, varies with the 

 species of parasite and the structure of the embryo, so that increase 

 of size appears to be the only change which can be universally pre- 

 dicated of parasites. Different species vary much in the dimensions 

 which they attain ; some stop short at a few millimetres in length, 

 others only after exceeding three or four decimetres (Ligula). 



B . C. 



A. 



FIG. 45. Entozoa in the second stage of development. A. Cysticercus 

 of Tcenia solium from the pig ; B. Cysticercus of Tcenia cucumerina from 

 the dog-louse ; C. Young form of Spiroptera murina from the meal-worm. 



If the embryos differ from their parents in form, they undergo 

 metamorphosis as well as increase of size. The organs that served 



