364 ZOOLOGY. 



not strictly correct, for there are not merely two generations 

 in the life-cycle, but many. As a general rule, in Metazoa 

 the reproductive cells or ova are unable to develop into new 

 individuals until they have been fertilised in other words, 

 have conjugated with other reproductive cells in the form 

 of spermatozoa. The germ cells in these cases are gametes, 

 cells destined to conjugate with other gametes. But the 

 reproductive cells of the sporocyst and redia develop directly 

 without fertilisation, although in structure and mode of 

 development they do not appear to differ from true ova. 

 This case of alternation of generations is, therefore, to be 

 distinguished from the metagenesis seen in Obelia, where 

 the sexual individuals are produced not by generation, but 

 by gemmation; not by the development of a single cell 

 separated from the parent, but by the proliferation of a 

 number of cells, of a portion of the tissues of the host, 

 including both ectoderm and endoderm, separation occur- 

 ring after development. 



14. Paedogenesis. The liver-fluke also illustrates 

 what is called paedogenesis, or the reproduction of young 

 individuals not fully developed. It is evident that the 

 sporocyst and the redia have proceeded a very little way 

 on the road of development which leads to a fully de- 

 veloped liver-fluke. They are even less developed than the 

 Cercaria, which is incapable of reproduction until it has 

 become adult. (Paedogenesis is derived from pais, child 

 or boy, and genesis, generation.) 



15. Coelom and Nephridia. Morphologically the 

 various stages of the liver-fluke illustrate the significance 

 and relations of the coelom or body-cavity and the nephri- 

 dia. At first sight the mature liver-fluke appears to be 

 destitute of a coelom : it appears to consist, apart from the 

 digestive cavity or enteron, of solid tissues in which the 

 generative and other organs are imbedded. But it is to be 

 noted that all the generative organs are saccular; they 

 consist of cavities containing the generative or accessory 

 cells, and these cavities are directly continuous with the 

 generative ducts. Now if we consider other animals, 



