206 THE DOCTRINE OF DESCENT. 



is exchanged for independent phases, signifies a rever- 

 sion taking place systematically in all individuals to the 

 once permanent condition of their progenitors. Of the 

 Trematoda or Flukes, and Cestoda or Tapeworms, be- 

 loncfincf to the class of the Platelmintha Suctoria, the 

 latter have diverged the most from their starting-point ; 

 their adaptation to life within other animals has rendered 

 the alimentary canal superfluous, and their generations 

 and transformations hence point less to their progenitors 

 than is the case with a number of other Trematoda, with 

 which many anatomical characters prove them to be 

 closely related. Both, moreover, share the characters of 

 their class with the free-living Turbellaria. From such 

 as these, that is to say, from forms approximate to the 

 present Turbellaria, the Trematoda and Cestoda must 

 be descended, and with this agrees the free roving phase 

 which the larva of the Fluke (Distomum) undergoes as 

 the so-called Cercaria, and previously as a rotating 

 spherical body. 



Many of the ciliated Nematoids, or thread-wormxS, 

 too, — the division which includes the Ascarides among 

 others, — have in their infancy a stage of independent 

 life, during which they cannot be distinguished from 

 the infantine forms of their m.ore numerous kindred, 

 which never adopt a parasitic life, and chiefly inhabit 

 the sea. The transition to parasitism, as recapitulated 

 by ontogenesis, was nothing more than an extension 

 to a new territory offering advantages of nutriment; 

 and on this point it is highly instructive to compare 

 the Nematodes with the systematic series of the leech- 

 like Suctoria (Trematoda), so excellently described 

 by Van Beneden. We here find all the transitions 



