208 



THE FLUKES 



In all flukes except those of the family Schistosomidse both 

 male and female reproductive systems occur in the same individ- 

 ual, and occupy a large portion of the body of the animal. We 

 are familiar with animals which appear to live almost wholly 

 to eat; the flukes are animals 

 which seem to live merely for re- 

 production. They are reproduc- 

 tive machines, all the other or- 

 gans of their bodies being devel- 

 oped only to a sufficient extent 

 to ensure the proper develop- 

 ment and maturity of the eggs. 

 The eggs proper and the shell 

 materials are produced by sepa- 

 rate glands, and sometimes the 

 canal for conducting the sperms 

 from another individual into the 

 body to fertilize the egg is distinct 



,. , , ! . -, T , ! FIG. 62. Heterophyes heterophyes, a 



from that which conducts the very small inte stinal fluke of man; A, 



eggS OUt of the body. The male adult; B ( X 350), spines from genital 



. , , ring; g. r., genital ring; g. p., genital 



System consists of two Or more pores; other abbrev. as in Fig. 74. X33. 



glands or testes for the produc- 



tion of the sperms, two sperm 



ducts which meet and enlarge into a " cirrus pouch" for storing 



the sperms until ready to be used, and a rectractile copulatory 



organ. All these complex sexual organs in a single animal which 



may be no larger than the head of a pin (Fig. 62)! 



Almost as soon as the fluke reaches its final host and assumes 

 its mature form, development of the reproductive systems be- 

 gins. Although both sexes are usually in the same individual, 

 mutual cross-fertilization generally takes place, the sperms of 

 two individuals simultaneously fertilizing each other. The num- 

 ber of eggs maturing in a single fluke is enormous, and while it 

 undoubtedly varies in different species and in different individuals, 

 the eggs are probably always to be reckoned in the thousands, 

 and sometimes in the hundreds of thousands. 



Life History. The life histories of all the flukes which are 

 internal parasites have much in common, and all of them undergo 

 a series of marvelous transformations from egg to adult. 



The fluke which is most thoroughly known in every respect 



shown 



above, x 500. (After 



