LARGE NUMBERS OF EMBRYOS. 81 



localities, and have been probably swallowed accidentally along with 

 the food. 



Local conditions also are of great importance in the distribution of 

 parasites, as has been shown by Melnikoff and myself, 1 in the case of 

 the dog-louse (Trichodectes), which harbours the young of Tcenia ellip- 

 tica (Fig. 45, B) and passes it into the dog. 



Although the life-histories of parasites largely depend, in the most 

 varied manner, upon the mutual relations of the animals that are 

 their hosts, it is also true that chance plays a very large part in their 

 determination. It is quite by chance, for example, that the egg meets 

 with its proper host, or that its host is subsequently devoured by some 

 other suitable animal. The more complicated, in fact, does the life- 

 history of the parasite become, the greater risk does it run of not 

 being able to complete its life-cycle. Millions of germs perish for 

 one that reaches maturity. 2 We have, however, already spoken of 

 this, and shown how it is compensated by the immense fertility of 

 parasitic worms. " If the eggs and embryos of Helminths always 

 attained to a suitable environment, the bodies of all men would be 

 absolutely full of tape- worms, Nematodes, and other parasites." And 

 it need hardly be pointed out that the lives of the parasites, as well 

 as of their hosts, would be greatly endangered by this. The compli- 

 cated life -history of the parasites serves as a means of checking their 

 too rapid increase, and their metamorphoses and migrations, therefore, 

 are of the highest benefit to them. 



Von Siebold has considered that those Entozoa found in the bodies 

 of the wrong hosts have " lost their way." 3 Nothing can be said 

 against this simple statement, but the conclusions which he has drawn 

 from it are by no means correct. 



In the first place, it must be remembered that any animal which 

 has wandered into a locality where its proper food cannot be obtained 

 a stranded whale, for instance may be said to have " lost its way." 

 The expression ought not to be confined to parasites, although perhaps 

 the occurrence is more general with them. Weinland speaks in the 

 following way of the life-history of corals :* "During the breeding 



Thomas and myself render it very probable that ruminants and other herbivorous mammals^ 

 devour Distomum hepaticum along with plants, to which Cercariae are attached in the*' 

 encysted state. R. L.] 



1 Archivf. Naturgesch. , Jahrg. xxxv., Bd. i., p. 62, 1869. See also Vol. II. 



2 A tape- worm has an average life of two years. It produces in this time some 1500 

 segments (see p. 43, note), each containing 53,000 eggs, the total number of eggs being 

 therefore about 85,000,000 ; since the number of tape-worms remains about the same, one 

 only of these 85,000,000 of eggs reaches maturity. The probability, therefore, against 

 each tape-worm arriving at maturity is as 85,000,000 to 1. 



3 " Handworterbuch. d. Physiol.," Bd. ii., p. 650. 



I 



