CHAP. XXXVI.] ALTERNATION OF GENERATIONS. 529 



The livers of various animals are infested with an entozoon 

 termed a fluke. The development of the fluke of the common 

 fresh- water snail (Limnseus stagnalis) presents us with a beautiful 

 example of the curious phenomenon we are now considering. In 

 the first stage of its existence it is seen as a creature (Cercaria) 

 swimming about in the water, and is provided with a tail. After a 

 time these cercarise fix themselves to the skin of the snail by means 

 of a circlet of hooks. The tail is cast off, and the body becomes 

 covered with mucus, which hardens, until a transparent case is 

 formed. This is the pupa state. Next the creature bores its way 

 into the body of the snail, and reaches the liver ; the hooks drop 

 off, and it possesses all the characters of a fluke or distoma. The 

 fluke develops ova; the ova becomes developed into worm-like 

 creatures, which inhabit the snails. The worm-like body contains, 

 as it were, a progeny, each member of which becomes the parent 

 of another generation. The original larvae are developed from a 

 perfectly spherical germ, consisting of granules. So that the early 

 stages of life of the fluke are passed in the body of a worm-like 

 creature; the next in the water, free; next, attached to the body 

 of a snail ; and, lastly, in a perfectly developed form in the liver. 

 Thus this creature assumes three distinct forms at different periods 

 of its existence, which, until these discoveries were made, had been 

 described as three distinct creatures. 



There are numerous other most striking instances among the en- 

 tozoa of this extraordinary change of character in the course of 

 development. It has long been known that the cystic entozoa (as 

 Cysticercus, etc.) are not provided with generative organs; but it was 

 reserved for Van Siebold to show that these entozoa were only the 

 imperfectly developed forms of species occupying a higher position; 

 and he has been able to prove that the cysticercus fasciolaris, which 

 is found in the liver of the rat and mouse, becomes developed in 

 the intestine of the cat into the tania crassicollis, the common tape- 

 worm of that animal. Kuchenmeister and Van Beneden have been 

 able to Demonstrate the occurrence of similar changes in many 

 other entozua. 



Another beautiful example of metagenesis occurs among the 

 members of a much higher class of animals insects. The ovum 

 of the perfect winged aphides, or plant lice, becomes developed into 

 an imperfect wingless or larval creature, in which no sexual organs 

 have been discovered. These viviparous but non-sexual larval 

 forms are capable of producing non-sexual descendants, exactly 

 resembling them, without the occurrence of any generative act, 



