50 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



with their constantly eiito-parasitic habit of life, and 

 they, like the endo-parasitic Trematoda, ordinarily pass 

 through different stages of their development in 

 different hosts. While the simplest forms, like the 

 Caryophyllaeus of the carp, exhibit no kind of jointing 

 or division of the body, and Ligula has the jointing 

 affecting only the internally placed generative organs, 

 most consist of a more or less large number of joints ; 

 Tsenia echinococcus having three or four,T. solium about 



Fig. 17. Tomia, showing the head and four suckers, 

 the unjointed neck, and the early joints (Strohila). 



a thousand, and Bothriocephalus latus having, it is said, 

 as many as 10,000 joints, and attaining to a length of 

 twenty-five feet (Fig. 17). As these joints increase in 

 size and approach maturity, the ova become fertilised, 

 and commence to develop ; on the joints breaking off 

 and escaping to the exterior, the ova within are set 

 free, and if eaten by the other host proper to the 

 tape-worm, they go through the earlier stages of 

 their development within its body. In these parasites 

 the digestive tract is altogether aborted. 



We have been carried away by these degraded 

 forms from the general line of development ; we return 

 to it, however, only again to find ourselves confronted 



