58 LIFE-HISTORY OK PARASITES. 



which are commonly said to go through no metamorphosis, the 

 resemblance of the young to the adult is more apparent than real. 

 In the majority of cases (in the Cestodes, Distomidie, Eckinorliynchus, 

 and Pentastomum} the differences are so great, that there is hardly 

 any point of similarity between the young and the fully formed 

 worm. (Figs. 35, 36, and 37.) 



It is not so much for zoological reasons, to complete our knowledge 

 of the organization of parasites, that these facts are brought forward, 

 as for the reason that the heteromorphism of the embryo is of the 

 greatest importance in their life-histories. Seeing that the structure 

 of an animal is by no means a matter of chance, but depends upon 

 the capacity for certain actions and modes of life, it is not surprising 

 to find that the embryos of Entozoa, which live under different 

 conditions from the adults, are different from them in form; and 

 these peculiarities are all the more important, because the fate of the 

 embryo is intimately connected with the character of its life-history. 



Let us consider the actual results of observation. It appears that 

 the history of the young parasites that have reached the exterior from 

 the body of their host, whether as eggs or developed embryos, may 

 follow one of two directions ; either the young leave the egg and live 

 in a free state for a longer or shorter period, or they remain within 

 the egg until it is taken into the body of a new host, where they are 

 then set free. In the latter case, there is no free-living stage, for it is 

 always the eggs and not the embryos that are found at large. But it 

 may be objected that it is impossible to draw a sharp line between a 

 living individual and a fully developed egg. This is no doubt true ; 

 but it must be remembered that the relations between the embryos 

 and the outer world are quite different while it is still enclosed within 

 the egg-shell, though an embryo just hatched can hardly be said to be 

 at a higher stage of development than a fully formed embryo still 

 within the egg. 



Whether the embryo of a parasite, when fully developed, be free 

 or not, depends in a great measure on the character of the egg-shell. 

 The latter, when thick and strong, imposes an increased resistance to 

 the exit of the embryo, and sometimes renders it quite impossible for 

 it to leave the egg by its own unaided efforts. This is effected very 

 often by the action of the gastric juice of the new host, which dis- 

 solves the shell, or makes it so weak that the embryo can force its 

 way out without special difficulty. My experiments J with the eggs 

 of tape-worms show clearly that the hatching of the embryo is some- 

 times merely a question of the digestive activity of its host. In some 



1 Leuckart, " Blasen warmer," p. 100. 



