138 ELEMENTARY ZOOLOGY 



alimentary canal thus forms a system of fine branches 

 closed at the tips, and extending all through the body. 

 The nervous system is composed of a ganglion or brain 

 in the front end of the body from which two main branches 

 extend back throughout its whole length. From these 

 main longitudinal branches arise many fine lateral 

 branches. 



Of the parasitic flatworms the tapeworms are the best 

 known. There are numerous species of them, all of 



FIG. 31. A marine planarian, Leptoplana californica. (From a living 



specimen.) 



which live in the bodies of vertebrate animals. In the 

 adult or fully developed stage the tapeworms live in the 

 alimentary canal, holding on to its inner surface by hook- 

 like clinging organs and being nourished by the already 

 digested food by which they are bathed. In the young 

 or larval stage tapeworms live in other parts of the body 

 of the host, and usually, indeed, in other hosts not of the 

 same species as the host of the adult worm. 



