130 



ORGANIZATION AND DEVELOPMENT OF ANIMALS. 



It is, however, extremely probable that the masses of cells 

 from which the Cercariae arise represent the rudiments of ovaries, 

 the elements of which develop parthenogenetically without the 

 addition of spermatozoa. The so-called germ sacs (Sporocysts or 

 Rediae) would in this case be larvae, which possess the power of 

 reproduction ; and the development of- the Distomeaa would conie 

 under the head of heterogamy. The Cercarise, however, represent a 

 second and more advanced larval phase. Provided with a motile 

 tail, frequently with eyes and buccal spine, their organization, save 

 in the absence of developed generative organs, presents great simi- 

 larities to the sexually mature adults into which they develop. 



This development, however, takes place 

 only in the body of another and usually 

 more highly organized host after the 

 loss of the larval organs. 



If the conception of a spore as an 

 asexual reproductive product be main- 

 tained, it becomes impossible in practice 

 to draw a sharp line between alterna- 

 tion of generations and heterogamy ; 

 since there is no test which enables us 

 to distinguish between a spore and an 

 ovum which develops parthenogeneti- 

 cally. On the other hand, if we inter- 

 pret, as we are justified in doing, the 

 so-called spores as precociously developed 

 ova, alternation of generations and 

 heterogamy can be clearly distinguished 

 from one another, since in the former 

 one generation is asexual, and in- 

 creases entirely by budding and 

 division ; while in the latter both 

 generations are sexual, though in one 

 of them the ova may possess the power 

 of spontaneous development. 



An essential characteristic both of heterogamy and alternation 

 of generations depends upon the different form of the individuals 

 appealing in the generations which usually occur in a regularly 

 alternating manner in the life-history of the species. But there 

 are cases in which two methods of reproduction may follow each 

 other in the life-history of one individual. This form of the 



FIG. 115. f, Young Distomum 

 (after La Valette). Ex, main trunk 

 of excretory system ; Ep, excre- 

 tory pore ; O, mouth opening 

 with sucker ; S, sucker on middle 

 of ventral surface ; P, pharynx ; 

 D, limb of alimentary canal. 



